I think you should take a look at vesting.
It protects the other founders in case one (or more) of the founders isn't working as hard to achieve their goals on the project. It does that by not giving everything upfront, so you decide the milestones each one of you will have to achieve and when you achieve them, you get some of the shares you're entitled, so for example, you'll work your way to your 40%.
Let's say your first milestone is to deliver the first beta (extreme simplification here), and you agree that it will get you 1% of your shares. If one of you walk away before a certain period (this is defined by the founding team), say 2 years, then you get nothing. This stimulates everyone to dedicate to the project.
If you don't do this, someone could leave after one month and get their full share, while the other founders will have to keep working and this person who left will get the fruits of all your effort without doing a thing.
What is founder vesting?
Vesting means that at the very beginning each founder gets his or her full package of stocks at once to avoid getting taxed for capital gains; but, the company has the right to purchase a percentage of the founder’s equity in case he or she walks away. [...]
In essence, vesting protects founders from each other and aligns incentives so everybody focuses towards a common goal: building a successful company.
This was in the first link I found when googling vesting for startups.
This article says:
Note: The legal aspect of vesting varies significantly from country to country. So, contacting a lawyer to make a consultation is generally a good idea.
I'm not giving any legal advice. In my country you don't need a lawyer to do that. I think it's good if you structure a written agreement based on vesting, just check your local laws.