I've had this problem follow me from rom to rom. I don't know if it started while I was still on Jellybean roms or after I hopped onto KitKat, but it has been with me for a while now.
When I'm going about my day, data will suddenly drop. The icon still displays 4G or LTE, but apps that need data won't connect and the browser just hangs. After rebooting, nearly always, the phone will be in 3G, even though the location has great 4G. It will work from here on out, or until it decides to crap out once again. It has done it with 3G too, though, the problem is not only with 4G.
In fact, I've noticed, especially as of late, that my service is awful. I can be in areas where I've usually had full bars, now I have just one bar.
On stock, I could just click PRL update or whatnot, but that's not an option anymore. Could it be that these sort of things are out of date? I'm really only familiar with flashing roms, not flashing kernels or radios.
Any help would be much appreciated, thanks.
Carbon Rom (Nightly - released 07/07/14, I believe)
Previous KitKat roms, same issue: