Why not change the 4 panels for Sunpower, or Panasonic, or was it jetson that are 50v?
Then if the inverter allows all the others could be wired on one string.
It may be more cost effective this way, without the potential of having to get back up on the roof every few years if you have any reliability issues.
It may need some careful system design, but then it should have had that in the first place.
Just a thought, that's all.