Should you provide any bonuses to employees whose ideas make a significant contribution to the bottom line?

We're just getting started with scaling our business and one of the things we want to implement is a 4/1 work day. 4 days are spent working, and 1 day is for individual projects (just like Google's 20% time).

If someone individual project becomes a success, should we provide a bonus to that employee? Or would that create pressure amongst the team?

That sounds like a can of worms. How much success does a project need to succeed for a bonus? If it keeps providing big returns, does the employee keep getting a bonus? How's the bonus divided when employees collaborate (often making unequal contributions)? What if a project doesn't directly contribute to the bottom line, but is clearly making a difference in another way? Etc.

What do you want from 20% time? If the goal is to boost the bottom line by letting employees use their own creativity and initiative, maybe a straight-up revenue-share would be a better option than giving bonuses. Then everyone's incentivized to support each other's projects, and it avoids a conflict between trying to do your job 4-days a week and trying to earn a big bonus. In a company revenue-share model, regular work and 20% work are all part of the same goal.

If it's personal-development for employees, a better way to go about it might just be to say that individual projects are one of the factors you look at when it's time to figure out raises, end-of-the-year bonuses, and promotions. It'd be good for internal movement; 20% projects could be a great opportunity for someone in QA to show they have the chops to be on the development team, for instance, or for someone of the marketing team to lead side-projects well enough you should consider them for product management.

If it's R&D, remember that people chasing money aren't going to do the same kind of work as people chasing answers. You're going to see a lot fewer experiments (which are often sparks for someone else to learn something that can improve the bottom line long-term) and a lot more efforts to generate some short-term revenue.

Identify what you want from it, and then align incentives to that.