The following could work if you are prepared to walk hard:
Day 1 - either Shepherds Cave area or ChiChi bush camp
Day 2 - ascend Mweni Pass, sleep in one of the 2 mentioned caves
Day 3 - traverse to Fangs Pass/Cave
Day 4 - descend Fangs Pass and walk out
I have done all of the above in some form or the other, so they are do-able, however, be warned, descending Fangs and walking out in a day is very long and hard...we did it once plus still drove several hours to get home.
If you want to do Rockeries, and make it up there within the first day, then this will allow you to descend Fangs Pass on Day 3, making Day 4 a lot more manageable.
As Stijn pointed out, if you are doing a Fangs-Rockeries combination in 4 days then it would be easier going up Fangs.
You can also do Mweni Pass in a day, but I would not readily recommend you try this.
Be warned, generally nothing in the Mweni is short or easy.
You can arrange for a taxi (ask Agrippa to arrange it for you the day before when you arrive) - they will either drop you off at Langwane's Kraal for Rockeries, or where the road ends near the split between the Mweni and Ifidi valleys, for Mweni Pass. This saves you 5km of walking on dirt roads. I've even arranged a pick-up at the end of a hike (several days in advance) before - and we were picked up on time! The price is negotiated. For a pick-up you'd have to make it worth their while obviously.
You don’t need to worry about permission to cross lands. The MCC is a community project already and benefits the 2 tribal authorities represented in the area. The locals are generally friendly and will even help you stay on the right trail - especially on route to Mweni Pass - it's a little confusing the first time when you pass through the kraals.
If you go soon, you may also meet the dagga planters. Again, they are friendly as long as you stay away from the crops

. Do be aware of potential (but very minor) petty theft by shepherds and hunters in the valleys. Mweni and Rockeries are very major smuggling passes.
The Mweni is a definite personal favourite - enjoy!