Could you please share what you feel are the disadvantages of the Mark IV?
MKIV's are good guns for sure, but they are not superior to MK II's. Yes, the disassembly is way easier, but taking down a MKII has never been a problem for me (no worse than a 1911) so I don't see the MKIV push button has a significant selling point. However, I get that the disassembly/reassembly of earlier models is NOT fun for the vast majority of their owners. The push button is THE major selling feature of the MKIV. But...
MKIV triggers are HEAVY! Out of the box, the three I've tried are likely in the 8 lb range. For $130+ dollars you can solve that with a Volquartsen Accurizing Kit, which also also eliminates the idiotic magazine disconnect--another "feature" you won't find on a MK II.
Reassembling a MKIV two piece safety is an adventure. Also, when shooting, some people are reporting that the safety lever sticks out too far on the MKIV. That safety lever is unnecessary.
Nor am I a fan of the fragile finish on the blued MKIV's, or the aluminum lower (on some MKIV models, not all), or the way the upper rattles (on many guns, the upper and lower don't mate up well. It doesn't affect functionality; it just rattles. Like an AR 15.)
The MKIV does has an improved mag release. The one I tried was pretty cool. You can hold the gun with the grip pointed up and when you press the mag release, there is a spring that pushes the mag. It will actually lift the mag about 1 1/2 inches up in the air. When the mag well is pointed down, like when you're shooting, pressing the mag release makes the mag blast out of the mag well! If you need rapid mag changes, the MKIV smokes the MKII. I just don't know how many people need quick mag changes on a .22lr pistol.
IMHO, dollar for dollar, you get a better gun for MKII money.