Problem with installing apps in app store and installous in iPod 4G. Pls help me?

My iPod touch 4g firmware is 4.2.1 but i cant dl anything from app store. The download finishes but it cant install. I always have to do it in iTunes manually. Moreover, i download an app from installous but it cant install... It says IPA error... I tried redownloading but it still doesnt works... I tried installing installous again, still doesnt work, reset all settings, doesnt works and reinstalling 4.2.1 but it cant (error 3904)... Pls tell me what to do.. I dont want 4.3... Ill really be grateful if you help me... THX