All UE games take a lot of RAM to even get past the UE3 splash screen, try making sure multitasking is clear, and if that doesnt work try a fresh restart. I've had issues with IB1 before loading up past splash and a restart fixed it for me (I'm a 3GS so not even a 4th gen)
At this point, all I can say is get used to it! Hopefully 2013: Infected Wars proves that games can be properly optimized fir our devices!