The Droid 4 has a smaller screen than his thunderbolt, but it has the keyboard. If the keyboard is important to him go that way. If not, I would definitely lean towards the nexus. I went from the thunderbolt to the stratosphere and had a hard time adjusting to the smaller screen. (someone will correct me if I'm wrong, I think the stratosphere has the same size screen as the D4). As stated above, the Stratosphere is really slow. I have no experience with LG smartphones but I have not heard good things.
Also coming from the Thunderbolt skin (sense) Motorola phones take a lot of getting used to, if he liked his thunderbolt I doubt he will like the way the D4 operates. I had a Bionic for the while and I never really got used to motoblur. (that's really just personal opinion, he may like it, a lot of people do)
Lastly, the Nexus has Google's latest operating system (the D4 should get it soon though).
Coming from a thunderbolt, if he could talk them into a Rezound, I think that would be his best choice. (It's HTC's successor to the Thunderbolt)