Some thoughts.
First, as a general rule, always get the most flexible, biggest tool you can. You will never regret having more capacity. There are obvious exceptions, but most of us can't afford a range of sizes all at once, or multiple expensive accessories.
Glacern's quality may have changed. I have their 6" reverse vise (615r premium), bought years ago, and am delighted with it- but there have been some threads about quality dropping. Check around. And look at the Shars equivalent. The premium designation now apparently means that it's "CNC-style". That is, absolutely, worth it. The value is that one can set up the vise on its side. Every now and then, that's incredibly useful.
A 6" CNC style (ground all over) vise is the go-to workhorse for the 1100. I like the reverse version, because I can mount it parallel to X, with no overhangs or interference with the column or enclosure while keeping the solid jaw at the left hand side (which after probing makes both X and Y moves positive- which I find convenient). This mounting preference seems to be idiosyncratic; some of us love this, many more do not. The CNC version can hold work from 0 to >14" by moving the jaws. You'll also want some soft jaws; make them, or buy (often cheaper, but not from GMT).
4" vises are just too small for everyday job shop use in the 1100, in my opinion. I have two, and use them from time to time, but the vise that lives on the table is the 6". In fact, I frequently put a 4" into the 6", rather than change out. For a small part, it's quick. Note that I'm never running multiples- everything is prototype or one-off.
Glacern's price for the 4" and 6" premium vises are identical, at 600 bucks. Given that, I'd buy the 6" premium.