Several people (including me) have had similar experiences with Li'l Gun in the .17 Hornet.
H4198 works well with 20-grain bullets, in fact in terms it's a very close second to 1680 in my rifle in both velocity and accuracy. The only problem is it doesn't run from a powder measure into those little necks easily, which slows down ammo production when cranking out lots of ammo. An extruded powder that does flow nicely, shoots well, and gets 3500 or so with 20's is Accurate LT-30. Like H4198, its also supposed to be temperature-resistant.
Have had my best luck with 25's using Accurate 2200, but RL-7 and H322 have also been very good.
If you have a chance to try any of the Hornady 15.5 grain NTX loads, they're also very accurate in my rifle, with an average muzzle velocity of around 3850 fps.
