Speaking as a recent owner of my 2nd concurrent Rumble (a Studio) I would add the qualifier that two Rumbles is only enough when at least one of them is a Studio or Stage. 'Cause, you know, with their 100 different presets, they each count for more than one
! [Edit to add: the LT25 also counts for more the one, but a little less than a Stage or Studio due to fewer presets]
Now, a Stage
and Studio
or LT25, with or without some other Rumbles, could be heaven.
Gravey, I think your 4 Rumbles is just about right for no Stage or Studio. So, ... ...
Rumble On!
.