How to update my Ipod touch 4g .
I'm having trouble updating my Ipod Touch 4g from software version 4.2.1. I lostit about a year ago and found it a couple of months ago. and it was working fine it let me download certain apps without it saying you need IOS 4.3 or Ios 5. and when I go into Settings>General, I see nothing sbout an available update. there are certain apps I cannot download because it says I need IOS verison either 4.3 or 5. Ive tried updating it fro itunes but it doesnt work, it says Itunes is the current version. Please help, I've tried everything. I've rested it and completley resetted it ( erasing everything.)
iPod touch, iOS 4.2