The uncompressed line-in 4 zone wireless limitation is a recommendation documented by Sonos. I don't have more than 4 zones so have no experience of it.

Quote
FAQ 1091 : Using the Line-In on a ZonePlayer
Changing the line-in encoding setting

The audio from line-in sources can be encoded to produce compressed or uncompressed audio. We recommend the following usage guidelines:

Use Uncompressed if:
• You want optimal performance with home theaters.
• You want to produce the highest quality sound for your multi-zone Sonos system (this selection requires more network bandwidth than compressed audio.)

Use Compressed if:
• You are connecting more than 4 zones wirelessly.

Meridian owner since 1992
Prime & PSU, Focal Elear headphones, Roon (ROCK 8Gi5 NUC), Explorer 1 & 2, F80. 200/203, various Sonos (via Roon).