4% seems somewhat reasonable, but it'll be interesting to see what effect this will have on the GH/s price. Coupled with the upcoming difficulty jumps, there should be some pretty heavy drops in prices.
After 2 more difficulty jumps, which is just 14 days from now, that 4% fee will become around a 10% fee. By the start of 2014 that fee will approach 30-40%. At some point data center costs will start to eat up all the expected revenue.
This is the end game for a hosting solution like this. Remember in the GPU era mining was largely a break even activity. Maybe slightly profitable for home setups, but negative if you ran in a professional DC.
Cex.io mentioned they would adjust the fees with the changing difficulty...I guess we'll have to wait and see...but it's in their best interest to keep it low otherwise people will just leave.