There is no way to only select 3G mode. But i have noticed i stay on 4g better.
I traded my Galaxy Nexus for a RAZR Maxx in favor of better battery life about a week ago. It lasted all day, sometimes 2. Last night I "upgraded" to ICS and now I have almost the same battery life as my Nexus did...
Current lineup: Note 3, iPhone 5S, Note 10.1 2014
Previous smartphones: Note 2, Galaxy S3, One X, RAZR MAXX, Galaxy Nexus, Rezound, Note, iPhone 4S, Bionic, Stratosphere, Charge, Vitality, Incredible 2, Thunderbolt, Palm Pixi Plus, Droid 3, Fascinate, Incredible, Droid Pro, Droid 2 Global, Droid X, Droid, Treo Pro, Touch Pro 2, Touch Pro, Q9m, XV6800, XV6900
Previous tablets: iPad Mini, Transformer TF300, Evo View, Transformer Prime, Transformer TF101, Thrive, gTablet
I actually think my battery is about the same if not maybe a little better with ICS. With GB i was getting 12 hours and the phone was at 15% or so and that was with light to moderate use. Today I have been on my phone all day trying to fix my contacts cause each one was in there 3-6 times. Was driving me crazy when I went to type in a contact for texting. I have also been searching the interenet for stuff and facebook and some email. Im at 11 hours 50 minutes and at 30%. I have not been on the phone this much since I got it so I want to say its actually better. I will find out for sure tomorrow when I go back to my normal use on the phone. I also have smart action features turned on. So when Im at home it disables my cell data and turns on wifi and some other options.