The Lowrance Mark 5's have a 5" screen, the Mark 5 pro is dual beam, the x-4 probably is dual beam but not sure and has a smaller screen. The Mark 5 pro is nice, you can scroll back to review something you might have missed and then fast forward to catch back up as well, The 5 has split screen, to view each frequency or zoom I think, even an old school flasher if you want that. The fish id feature is nice, it displays a small fish instead of the arches if you choose that option in the menu. BUT the fish symbol is VERY VERY small and hard to see from 3 feet away. The Mark 5 pro has several other features as well, is a decent unit for the $$, but would advise you to go to BPS or some store that has all of 'em on display, start punching buttons and asking questions, then choose the one you like best, is easy for you to operate since it's your boat. The Lowrance DSI units do NOT have the traditional 2d sonar, but I think the HB ones do but not sure.
The HB units probably have some or most of the same features, make sure you compare apples to apples, either brand should do you good.
GO BIG ORANGE !
I meant to behave, but there were just way too many other options available at the time.