The 5B is electrically almost identical.
Steve explained the reasons for releasing the new 4 band rig -
1.The 5B code does support 80m but the rig needs modification to work on that band (RX BPF and TX LPF). A number of MTR5B kit purchasers stated the desire for 80m operation and actually did a group buy of filter components. I think this gave Steve the impression of a demand for 80m capability in a trail rig, especially in Europe.
2 The MTR 5B from LNR (not the limited initial kit) used an LCD with a blue back light, this was unpopular, especially as the window in the case was too big, allowing bright light to be emitted around the edge of the display.
3 A source of 4 position switches were found which makes band switching much easier, using one bank of three switches, rather than two banks.
I think the MTR 5B is a cool rig, I'm sure the 4B will also be popular. The LCD on the 4B is only 2 lines, whilst on the 5B, the display has 4 lines. The characters are quite small on the 4 line display, so I guess they'll be a bit bigger on the 2 line display, making them easier to see.
A thing to note is that the 98 initial kits (not LNR Precision) has a different LCD to the later LNR Precision version.
The 2 line display apparently has a different pin-out to the 4 line displays, so retrofitting is not possible to the MTR5B without serious hacking.
IMHO, the MTR4B is just as valuable as the MTR5B due to the nicer case and the different LCD might be better for some people.
I really like my MTR-5B, it's a lovely rig!