Does anyone know the difference between the two Radios. Is it just the Receiver?

The biggest noticeable change is the main navigation button. The new design is a major improvement, it's more comfortable and is easier to use. I just sold my old 4PK and bought the 4PKS Friday. they did add some new mixes specific to crawlers and the system now recognizes Life batteries. As you stated the receivers are almost half the size and they decreased the latency even more.

He is right, the navigation button and the receivers are biggest difference. From what I understand tho you can use the new receivers with the old radio, you just dont get the benefit of the same speed as the new radio. I opted to get a slightly used 4pk just because the pricetag on a new one is a little out of my pay-grade and it works just fine.

Thanks for your information.