I don't have experience with the 4MS... but I'm willing to evaluate one! :)
A module you might want to try is the Expert Sleepers Disting Mk4. It's under $200US. I hate the interface. But it's full of different routines for reverbs, compression, resonators, etc. It's great for trying out different effects and figuring out if you'd like a dedicated module. Your Erica DSP will have some overlap but not too much as the Erica concentrates on just effects and not processing.
Another benefit you have is the ES E9. You can use your DAW as a virtual module with audio and CV flowing between. While not convenient, you can explore different effects and decide if you'd like something in hardware.
If you're keeping the MiniBrute, then you have access to additional traditional envelopes, filters, etc., I'd hang on to the Neutron for a while as it can serve as a utility box of modules until you're ready to retire it.
The biggest lack I see are mixers and attenuators/attenuverters. Most people overlook them. You're missing out on the power of modular if you can't submix signals or attenuate/invert them... both audio and CV. If you have an SQ-1, then the Voltage Block might be a bit redundant vs. adding some more utility modules. You have to make that call, though. Be aware that the Ornaments and crime does have some basic sequencing abilities in it that will cover a portion of what you get out of the Voltage Block.
Also... order your modules in stages as you are meaning to fill this rack. As you get to know each of your new modules it might influence where you go in finishing off your rack... priorities will change. So don't be in a rush to fill this thing.