Can I remove previous versions of iphone support in DeviceSupport folder in Xcode 4?
Yes you can remove SKDs prior to 4.0 in Developer/Platforms/iPhoneOS.platform/DeviceSupport it's just old stuff.
For the subfolders in Developer/Platforms/iPhoneSimulator.platform/Developer it's all the devices version that you will find in iOS Simulator in the menu Hardware>Device>Version. It can be sometimes usefull to keep them to test retrocompatibility (But remember that a test on a real device is still better). But if you want you can also remove these SKDs.