The enclosed one is a "deck mount" you can put anywhere. The open one is to go in the "lower portion" where the original one was. They are the same converter for how they work. You want the deck mount.
Make sure that cupboard gets air to cool the converter
The 4 means it has an extra stage trick to do a shot of 14.x every so often after it drops to the 13.2 storage stage. You will never see that doing 13.6 all the time using the RV on shore power, so a PM3 is the same for you. Besides that, the value of that shot of 14.x every so often is dubious. More of a sales gimmick IMO.
I have PowerMax converters and like them. I mostly like the ADJ version with the adjustable voltage so I can meet different battery specs that call for various voltages. In your situation, the basic PM3 would be fine.
