Thanks for the report.
With Teoz, you should not use "par2".
I think you can have the very same result with:
@startuml
!pragma teoz true
box "group 1" #lightblue
Participant P1
Participant P2
end box
' I would like to insert an horizontal minimu space constraint here
box "group 2" #lightgreen
Participant P3
Participant P4
end box
P1->P2
& P3->P4
@enduml
Is this what you are looking for ?