The Mortar is basically a beadless gator, it's very overstable with very low glide. As jjmiller stated it will be like a putter in distance. I use mine almost exclusively for approaches. I don't really throw it full power honestly, there are so many other ways to get 300 feet of distance more reliably.
The MD4 has slightly more HSS than the MD3 or Roc3, with about the same LSS. It will have acceptable midrange distance but with slightly less glide you won't throw it as far as a Wasp, MD3, Roc3, etc. It will fade reliably and handle all the power you can throw at it, but its not a Gator/Mortar. It's more like a Verdict, although I have only thrown those a handful of times.
I bag the MD4 on really windy days and lean on my MD3s the rest of the time. You could use it as an approach disc, but it won't perform the same hard finish of the mortar/gator. I see no difference at a powered down 200 foot sidearm approach between my MD3 and MD4, but the mortar finishes easily 10 feet further on line.