As others have said, the lack of religion is not merely the lack of places of worship and the mentions of gods. Belief is part of the fabric of any group. The legends, fears, hopes, understanding, etc.
1. Blessings and curses. Do your people believe in luck/fate/fortune? (As an idea, not a being) If not, then your 'good lucks' will more likely be 'don't give ups' or 'you can do it.' and so on. A comment above mentions the effect on curses.
2. How do your people explain the world around them? If purely with science, then how are difficult concepts broken down for the less educated? Does the belief in science lead to any reverence for those who seem to understand the secrets of the world?
3. What are people afraid of? Why? How do they protect/comfort themselves?
4. What is the ideal person? Who do people strive to be/be like? Why? Or, if not one person, what are the traits commonly agreed to be good and desirable?
5. Seconding Tyger's comment above
6. Superstitions?
7. How do people explain disease? And how did they before that explanation?
Etc.
These aren't things you'd necessarily need for a story, but since you're world-building, they are some of the things you'd probably want to consider.
To answer your other question-- If I was reading a story that made no reference to religion, I'd think nothing of it. Religion isn't always necessary to a story and shoehorning it in would be silly. (The blatant lack of religion, however, is noticeable and, frankly, silly. It is one thing to leave it out and quite another to constantly draw the reader's attention to the fact that you left it out).
If I was reading world-building notes that made no reference to religion, though, I would definitely comment.
As for having four different species and not wanting to make four different religions...
1. Do any of them have a religion with customs that would be noticeable throughout the average day or week?
2. Are any of their religions opposed in a way that they would bicker or leave the discussion of religion aside in order to preserve peace?
3. Do any of them practice a religion that would require them to abstain or indulge in certain drinks, foods, or practices?
4. Do any of them see religion as the source of their power or understanding?
5. Do any of them wear symbols of their religion? Are they easily seen or would they only appear under certain circumstances?
6. Does religion affect how any of them speak?
And then, to build the religions themselves:
As a single person and not a whole group practicing the religion-- who is the primary recipient of their belief? (A god or goddess (the only option or one of many)? Nature in general or specifically (a mountain, the sun, a star, the ocean, trees, etc)? Ancestors (a patron or in general)? A concept (luck, fate, love, etc)? etc) // Why do they believe? (grew up? chose? converted? part of life and not really thought of as any more special than cooking or sleeping? greatly important? Road to Damascus moment?) // Finally, is their religion typical of their people or not? How so and why?
You don't really need to create a full religion for each of the four characters. You just need to figure out how it would affect the four of them and when it may be visible. Later, you can color in those areas as they become relevant.
no subject
1. Blessings and curses.
Do your people believe in luck/fate/fortune? (As an idea, not a being) If not, then your 'good lucks' will more likely be 'don't give ups' or 'you can do it.' and so on. A comment above mentions the effect on curses.
2. How do your people explain the world around them? If purely with science, then how are difficult concepts broken down for the less educated? Does the belief in science lead to any reverence for those who seem to understand the secrets of the world?
3. What are people afraid of? Why? How do they protect/comfort themselves?
4. What is the ideal person? Who do people strive to be/be like? Why? Or, if not one person, what are the traits commonly agreed to be good and desirable?
5. Seconding Tyger's comment above
6. Superstitions?
7. How do people explain disease? And how did they before that explanation?
Etc.
These aren't things you'd necessarily need for a story, but since you're world-building, they are some of the things you'd probably want to consider.
To answer your other question-- If I was reading a story that made no reference to religion, I'd think nothing of it. Religion isn't always necessary to a story and shoehorning it in would be silly. (The blatant lack of religion, however, is noticeable and, frankly, silly. It is one thing to leave it out and quite another to constantly draw the reader's attention to the fact that you left it out).
If I was reading world-building notes that made no reference to religion, though, I would definitely comment.
As for having four different species and not wanting to make four different religions...
1. Do any of them have a religion with customs that would be noticeable throughout the average day or week?
2. Are any of their religions opposed in a way that they would bicker or leave the discussion of religion aside in order to preserve peace?
3. Do any of them practice a religion that would require them to abstain or indulge in certain drinks, foods, or practices?
4. Do any of them see religion as the source of their power or understanding?
5. Do any of them wear symbols of their religion? Are they easily seen or would they only appear under certain circumstances?
6. Does religion affect how any of them speak?
And then, to build the religions themselves:
As a single person and not a whole group practicing the religion-- who is the primary recipient of their belief? (A god or goddess (the only option or one of many)? Nature in general or specifically (a mountain, the sun, a star, the ocean, trees, etc)? Ancestors (a patron or in general)? A concept (luck, fate, love, etc)? etc) // Why do they believe? (grew up? chose? converted? part of life and not really thought of as any more special than cooking or sleeping? greatly important? Road to Damascus moment?) // Finally, is their religion typical of their people or not? How so and why?
You don't really need to create a full religion for each of the four characters. You just need to figure out how it would affect the four of them and when it may be visible. Later, you can color in those areas as they become relevant.