Insight genesis is Lowrance's online mapping solution, which can create maps in the same way as ReefMaster for display on Lowrance units.
In answer to the first question, pretty much any Lowrance unit will be able to display RM and IG generated maps, and the Elite 4 is compatible.
The main difference between RM & IG is that IG is very much a "one click" solution. You simply upload your sonar logs and receive a map in return. It's very easy to use, but as a result is less flexible than RM. RM can generate exactly the same sort of maps as IG, but a lot more user input is required; for example, you will need to supply your own lake shorelines, decide on contour intervals and interpolation limits, choose palettes etc. More work, but more flexibility as to the finished product. RM also enables you to export data in formats other than maps for Navico units - e.g. Shapefiles for use in other GIS applications - and has an optional sidescan mosaic module that can be used to view and export sidescan mosaic imagery. Which is right for you depends really on what you want to get out and how much effort you want to put it. A lot of people enjoy getting involved in the finer details of creating maps and it's also true to say that many people don't! Since you can trial both for free, I'd suggest giving both a go to see what works for you.