I used a Standback for years and they're great. If I remember rightly, they say it'll easily support a 4x12 so it should do the job for you just fine. First impressions are that it doesn't look that strong, but don't be fooled, they really are. Mine still looks as good as the day I got it. And they fold up flat and will easily fit into a bag or the back of a rack case. I got mine to improve my stage monitoring with my old single 1x15 cab set up, and it did the job brilliantly. Instead of the backs of my legs getting all the bass, once the cab was tilted I could hear everything. But, if you have a separate head you won't be able to put it on the cab as it'll slide off. I had to stand my flightcased amp on it's side next to the cab. No big deal for me, it still did the job just fine.
I don't use my Standback any more but I'll always keep it, just in case the need arises. A great accessory.