First off, absolute kudos to you for recognising that this could be a potential issue for you down the line. Most co-founders in your position would simply say "let's all get 25% each and work the rest out later". Unfortunately (for me, but good for those I advise) I've been in a situation in one startup where a lack of communication in the early days set us up for the ultimate downfall of founder fall-out over aggressive diluting.
So here is my take. The business is worth nothing right now and you all contribute in different ways. Each founder can make a list of their milestones of what they believe they can achieve in 30 days, 90 days, 180 days and 365 days time.
Of course you will all be dependant on each other (oh yeah, they call this 'interdependence'...), therefore you all need to agree on the rate of progress in a 'plan B' scenario. For example if a designer said they will get the prototype designs out in the first 15 days but then can't do any more design until the devs do a prototype and have launched the MVP...well the designer needs to broaden their scope to ensure they are adding value in the case where the devs don't deliver on time.
Ok, at this point everyone agrees on what should be achieved by when. Now you can break up equity to be earned over time. If there is a fallout or someone can't work anymore, they simply vest what they have earned but know that it will be diluted over time.
The complications arise are when it comes to raising money. If someone (or multiple people) are putting in money, then they should reasonably get their equity portion for the cash up front and then earn their 'sweat equity' just like everyone else.
The exact proportions are totally up for grabs but the principle is this:
"No one knows the future. We all love each other but the universe is crazy and sometimes shit happens. Let's be fair now so we just have to follow the agreement later."
I would suggest that everyone earns more equity over time such that if someone stopped contributing they could maintain their share by contributing cash instead.
How to avoid all the heartache? Build an MVP, ship it, get customers to buy it...all within 30 days. Fail fast and move on, just have principles agreed with your cofounders up front for when you find a winner.
If you need specific advise, schedule a call and we can talk further.