All very great advice here. The folks in this subforum have always been warm and caring people with great advice! I don't own any Survive! knives yet as I am still awaiting my orders to arrive but I do have knife in a similar size and built to the 5.1 so here's my take.
All Survive! knives are built tough and built strong. There really is nothing they cannot do. However, with a smaller knife like the 4.1 you will need to put in more effort to get bigger chores done. For example, you can't chop wood with a 4.1 because it does not have the mass to do it, but you can beaver cut through it. It just takes longer than straight chopping through.
That said, if you are going to own only one, make it the 5.1. It is probably the most balanced knife designed with enough heft to do some light chopping if need be, enough length to do some light batoning, and not too heavy to do some finer camp craft works.
I have a feeling that after you own the 5.1 you may be wanting to add the 4.1 into your collection. From what I see, Survive! knives are just that addictive.