If you are certain that you only want monitoring on 4 tires, the new solar Tireminder seems like a good bet.
I have gone though this back and forth, and am likely going to order the EEZ TPMS with 8 sensors so I can monitor the tug tires as well. I almost bit on the solar Tireminder, but decided I wanted to monitor the truck tires.
Do a forum search for "TPMS" and you'll see several threads discussing different brands. On my 21 I installed the wireless transmitter underneath the shelf over the bed and the receiver on the rear glass of my Tacoma with VHB tape. Works great without signal dropout issues.