I'm not a cell phone expert either, but I don't think upgrading your phone from 3g to 4g is going to help your talk reception. Might with data, but I don't think with talk. It's all the same format (i believe) between phones for talk.
If you had great service before and now all of a sudden you don't, you may have lost the tower that was giving you that good reception.
I used to use those nextel phones for work and my service was great near my house. One day, I went from all bars to barely having one. I called the company and found out the tower that I was getting my service from no longer provided nextel service and that's why I wasn't getting any more service.