Floored by the kind and detailed responses; thanks to both of you! An adjustment follows below (with optional FX...):
To Chinipira:
What is a minimum viable synth that is fun enough that you won't be disappointed and sell the whole thing in frustration.
You summarize well my initial objective!
I don't know Permutation but instead of that I'd recommend Pamela's New Workout.
Pam is indeed on my radar. I suppose it has more functionality, which should be a priority at this point. Still on the fence.
I love effects, get a Happy Nerding FX Aid (maybe XL) if your budget allows. Filters are also great but you can start without one. You can even create filter-like effects with Maths.
Good idea about the effects (though +1 module). My thoughts exactly on the filter.
Also, if you have other hardware, like a drum machine, find a way to connect them to your modular. For MIDI there are many modules, like the Mutant Brain. For connecting to a DAW the Expert Sleepers ES-8 or ES-9 might be the best, but there are other options too.
I do have an MPC 2000XL lying around... eventually will connect it to the modular. Trying to stay off the computer.
To bopodoq:
3) VCA, I second MI Veils and Intellijel Quad VCA. WMD has the Javelin which is a full ADSR with integrated VCA and separate envelope output (great 'bang for the buck').
There appears to be strong consensus to have a quad VCA and a separate output module. I have made that adjustment, even though it breaks my 4 module constraint.
To both of you:
With respect to the Cloud Terrarium, I am not dogmatic and indeed followed uninhibited excitement on this one. While seemingly controversial, I like the screen. The 4ms Ensemble Oscillator also interested me. A complex oscillator like an Odessa/Endorphin.es Furthrrr/etc appears to afford more functionality though, and can implement some of the ideas suggested by bopodoq.