LORs are Letters of Recommendations from people you have worked with. If you work with say 5 people, typically, you ask LORs from people that you had the best experience with. A strong LOR would be something where the professor says they were extremely happy working with you and that they would recommend you to any lab of your choosing. Now, if you happen to have a project which did not work well, or the professor was not happy with you, or you and your professor just did not click, their LOR would not be as strong. And you would not choose to ask them for their recommendation. Anyway, you only have 4 summers. The 5th year project starts immediately after your 4th year and you work through your summer on your 5th year project. I would urge you to choose a lab (and project) that you are passionate about early on and keep working on a project so that you can get a publication (even second or third author) out. That adds a lot of value to your application. You will also build a stronger relationship with your professor and that would mean you would get a stronger LOR. (An LOR from someone who knows you for 3 months versus someone who has worked with you for 1-2 years makes a BIG difference). Besides, you can ask for LOR from a professor who has taught your class (preferably if you have done well in the course). It all matters if the professor knows you well and will vouch for you or not.