The Cat B15 has asmaller screen, and can only do 720HD, not the 1080HD of the Gnex 4. As mentioned, 512MB RAM vs 2GB RAM, so just being able to download and install software is a bit limited on the CAT. The expandability via MicroSD card is in the CAT B15's favor. The Cat does NOT have NFC, so you can't just tap phones and share info (would be useful on a construction site for sharing the latest orders for the day, changes to a plan, etc), nor could you use the Google Wallet app because of it.
Yes, the GNex 4 is newer, more powerful, more stylish, and so much more. Yet considering that my first smart phone fell out of my pocket and took a swim, my second one had the screen bazzel falling off, and the last one the screen cracked on me, I think the CAT B15 is the right choice for me. I hate giving up the added RAM, while there is no 4G service where I live and work so that is not a concern for me. The added speed of a quad core vs dual core, and 1.7Ghz vs 1.0Ghz would make a difference if I played games on the phone, yet I don't, not even Angry Birds.
I think you have to decide for yourself, and if 4G is that important, then go ahead and take the chance, wrapped in a hybrid case. I would love a Google LG Nexus 4, and I may get one yet, just save it for personal use, while the Caterpillar would be for work only. Just don't like having to spend $700+ to get both, add a second wireless plan, and keeping one near by while carrying the other (I am on-call in case of broken water pipes, etc).
Yes, if you look at the specs then the GNex 4 is more powerful, while the CAT B15 is the right choice for hostile conditions.