The best replacement for rails 4 is rails 3.

The best alternatives to rails 4 are rated as following:
★★★★★  rails 3
★★  django 1.5
★  sinatra

Your alternative for rails 4 is not listed? Suggest!
Speaking about rails 4, one probably shouldn't forget about mongodb.

Other search terms related to rails 4 and their ratings are:
★★★★★ mongodb
★★★★★ bootstrap 3
★★★★ ember
★★★ devise
★★ mongoid
You have in mind a companion for rails 4 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of rails 4 in all years. Click on graph for more.
rails 4
rails 3
django 1.5