I have experience with regular synthesizers. I also have experience with semi Modular synthesizers (mother 32, neutron, DFam). I sold all my stuff a while back because of a financial hardship. I have been missing the noodling that I had with my mother 32 and Dfam and neutron. I always found the sequencers on stand alone synths to get boring and repetitive. My plan is this rack.

Basic three voice jams with a Keystep pro (and controlling VST for drums)

Learning to have slow ambient (maybe self generating as I learn)

Looking to spend under 4k
Case will be a mantis
Advice would be appreciated

A few observations...

First up, the Brains supposedly played a part in why Emilie is shutting Mutable Instruments down. Yes, the Plaits (which is what that actually is) was an "open source" project, but having the module basically stolen had to have hurt.

Secondly: are you trying to build two different synthesizers in the same cab here? It seems that way...and I can assure you that you WILL wind up with two different synthesizers, both of which will be pretty compromised in terms of capability.

Third: have you ever had any hands-on experience with those Mutable clones, such as the After Later stuff in this? If not, I'm pretty safe in saying that the tiny knobs with little clearance all mashed together WILL drive you nuts.

Fourth: mults. This build is way too small to accommodate them. In the current version, you lose 8 hp on those alone. You're far better off using the 8 hp for actual functionality, and then using inline mults and/or stackcables to deal with the stuff the mults are there for.

Hmmmm...gonna futz around with this...

[time passes]

OK...got it.
ModularGrid Rack
So what's going on here? Let's see...

TOP: Starts off with a ring mod/slew limiter/sample and hold, then there's four VCOs from Uli's System 100 clones. After that are six VCAs...and why there's six and not four is because I opted to go with a module that gives you two extra VCAs that you can use in cross-mod tricks between the four VCOs. You'll see a little more of that in a bit...anyway, after the VCAs are two VCFs. One is your Tiptop Steiner Synthacon clone, which in this context is more of a "lead" VCF due to the wilder nature of that filter. So as a counterpoint to that, I put in one of G-Storm's Korg Delta VCF clones. These are based around SSM chips and the same filter architecture was also used in the Poly61, and as such, it's the "nice" filter...very smooth, great for pad-type sounds. And then, yep, more VCAs, these being for controlling the VCF output amplitudes. And again, two of these VCAs are for other uses as needed. After this, we get into the mixing stage, where we first see the FX units...both from Tiptop, their Echoz and Zverb...and then a four-in stereo mixer. Now, look at the master section of that, and you'll find the FX send/returns...which are set up perfectly so that you can feed mono sends to the Tiptop FX and get them back in stereo, just like the TexMix is set up for. Oh, and the TexMix has VCA levels on each input strip...so even better control!

BOTTOM: First off, the sequencer section, where I chose a Tiptop Z8000...which is an interesting device in that you get aspects of Buchla and Serge-type sequencers as well as more "normal" sequencing duties. This can either feed the Behringer Sequential Switch (which leaves you with an extra sequencer channel for controlling other things) or the Klavis CalTrans quantizer directly, since it has four inputs on its own. If you want long sequences, use the Behringer to string patterns together and then send its output to the CalTrans. And that can be fun, since you now have three spare channels which can "quantize" modulation signals into semi-random tesselation figures. Noise gen is next, with its own S&H plus a random voltage generator. And then the LFOs...four of them, via Xaoc's well-loved Batumi (with the Poti expander). Maths is next, then a Frap Tools 321 and a Happy Nerding 3xVCA...and those are for altering/modifying/wrecking other modulation sources to get MORE modulation signals. Then at the very end, Xaoc strikes again with their Zadar (and Nin expander) quad EG.

This should keep you busy for a hot minute. I left several "open-ended" bits in there (as noted above) to give you a number of different options for control, modulation, and the like. And the Z8000 has the ability to fix that sequencer aversion...it's a rather complex critter, hooked in with the ability to string sequences and then quantization for each oscillator, meaning that if you wanted to, you could send each Z8000 channel to a channel on the CalTrans, which results in a CV line per VCO. Yeah...four-voice sequencing! And with the current price of the Mantis, you are right at your $4k budget.