My Mesa has a multi watt switch, full power, half power, and low (quarter). The manual says if you are running on half or low power with an 8Ω cab, they recommend using the 4Ω jack, for maximum headroom, tighter attack, less roll off etc. and there's "safe mismatching impedances" which is always using the one step lower jack with a higher cab i.e. 8Ω jack with a 16Ω cab or 4Ω jack with 8Ω cab. I haven't experimented with it much on full power but I believe that running a safe mismatch makes for a darker tone, and from what I've heard the amp actually runs cooler.
So when you go the other way, lower impedance cab and higher impedance amp, you will probably get smoke... and an amp tech bill
Search for "safe mismatch" on here.
http://mesaboogie.com/media/User Manuals/bassStrategy_170612-web.pdf