RD50 combo 4 ohm cab question

I thought I read that if I put a dummy plug in the main speaker out jack and plug my 4 ohm cab into the extension jack that the amp will be okay with a 4 ohm load. Is that right Steve???

Almost!
Go to the Manuals section of this site and look at the Owner's Manual posted for the RD50. On the page outlining the back-panel features under "R. External Speaker" it says:
"Note: A 4-ohm load may be safely driven if connected to the Speaker Jack and an open standard phone plug is inserted into the Ext. Speaker jack."
Steve

Ah HA! Thanks Steve!! I knew I read it somewhere!!
You may laugh but when I go to jams sometimes I fill in on bass. I want to try the RD50 out on bass with a 4 ohm cab. I own a 1x15 bass cab with one of Avitars semi custom Eminence Neo Delta speakers that is 4 ohms. The cab is super light (40lbs), very efficient and rated a real 300 watts RMS. The RD is soooo clean I think I can play bass through it quite effectively for quieter jams or gigs and still have a guitar amp if I switch off or play harp.
As a side note, unfortunately I don't think they make that exact speaker any more. This one Dave at Avatar had made so it goes to 3500hz which is unheard of for most 15's. It is sonically very flat which is okay too. Of course its pretty medicore for guitar - no mids & highs. On bass I can play my little Hartke bass head (200 watts) through it almost dimed and it can take the thrashing as long as Iam not clipping. I think the RD50 and this cab will be great for a dual rig...better than my Blonde Bassman that breaks up on 4.