The best replacement for ios 4 is ios 5.

The best alternatives to ios 4 are rated as following:
★★★★★  ios 5
★★★★  ios 3
★  froyo
★  android 2.2
★  android 2.3
★  gingerbread

Your alternative for ios 4 is not listed? Suggest!
Speaking about ios 4, one probably shouldn't forget about iphone 3g.

Other search terms related to ios 4 and their ratings are:

★★★★★ iphone 3g
You have in mind a companion for ios 4 and it is not yet here? Propose!

Google Trends

Scale is based on the average worldwide traffic of ios 4 in all years. Click on graph for more.
ios 4
ios 5
ios 3