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.