Okay, so you have the starting weight of each of the team members, S1, S2, S3 and S4, and then the finishing weight of each of the members, F1, F2, F3, and F4. Okay?
Add the weights together: S1+S2+S3+S4=S5. F1+F2+F3+F4=F5. Yes?
[1-(F5/S5)]x100 = percentage of weight lost.
An easy example: Everyone starts out at 200. 200+200+200+200=800. They all finish at 194. 194+194+194+194=776. 776/800=0.97. 1-0.97=0.03. 0.03x100 = 3% weight lost!
Am I making sense there? Yes, it is fair if one team has more members than the others, because even though there's more weight to start and more weight lost, there are more people to spread it between.