An ER4 can be pretty restricted out of SOU I believe
How so?. I know that British Regional Airlines operated the 49 seat ER4 (ERJ 145EU variant) to destinations such as BRU, EDI, GLA, FRA and ZRH without restriction. The only time I have heard about any restrictions was when Brymon introduced there ER4's and I believe the restrictions was specific to the 50 seat ERJ 145EP variant.
I think we will see the BRU switch to being operated by a ER4, with the lunchtime flights being dropped and the aircraft being used to operate the STN.
Another option would work is to base a ER4 at NCL which could operate NCL-SOU-NCL-STN-NCL-SOU-NCL-SOU-NCL. The aircraft could operate weekday service as shown below.
BE6xxx NCL 07:00 SOU 08:15
BE6xxx SOU 08:50 NCL 10:05
BE6881 NCL 11:00 STN 12:15
BE6882 STN 12:45 NCL 14:00
BE6xxx NCL 14:45 SOU 16:00
BE6xxx SOU 16:30 NCL 17:45
BE6xxx NCL 18:15 SOU 19:30
BE6xxx SOU 20:00 NCL 21:15