For a stand to support a 4' tank it needs to transfer most of the weight strait down to the floor from all 4 sides. It looks like that stand just has a few round feet to hold all the weight.
I think a 55g without any problems and MAYBE a 75gal with some additional support (maybe just some better pictures to see what is going on there.)
my biggest concern with that stand is 9/10 its made with MDF, which is great when its dry but the second it gets a little damp its going to get ugly and fast.
My first 75g tank came with a stand made from MDF. It's not too much of an issue as long as the laminate on the MDF stays on and you don't let the water pool on top for to long when you spill.
If it is made out of partical board I would not even consider it. Partical board does not hold up to water well and splits easily when there is too much weigh in one spot.