Theres alot of ways of doing it but one way I use is to use
blenders “scene” functionallty, the drop-down list on the top menu bar.
Create a box then flip the normals, uv-map the images to the appropreate
box side (check the image below). Then make a new scene block/empty
and then link the first scene as a reference in the"scene" tab F10
(the drop-down list under the FTYPE browse box).
Then change to texture mode ALT-Z, when you want to model you will have
to change the draw type to “solid” or"wire" in under"object" F7, of
the object your modeling otherwise its hard to see what your doing in texture mode.
… or you could just use different viewports for each…
the unque background image for each view (front top left back bottom right camera) is said to be possible, and I would expect it to be coming
(I have not heard of or seen it in 2.33 though)