do I need four BMS's

[Stefanseiner]

please advise which BMS to useexcept Batrium which I can't afford.

I am using separate BMS with included active balancer for my separate powerwalls.
There are models from 60A to 200A continuous power (twice for peak).
Not connectable all together but highly adjustable and can be monitored by app

adjustable Cheap Chinese Active Balancers

 

[daromer]

DIY BMS is one of the most affordable in terms of good bms but it is DIY.
You dont need active balancers unless you have either have issues with your battery pack and that often from neglecting best practicing building. Its better to fix balance issues than masking them.