Well... Cinder Blocks can hold up the roof of a building... so I doubt a tank small enough to fit in your home would be too big for them to support the weight of...
I would put them at more than simply the four corners though... I would also suggest putting a layer of styrofoam between the tank and the blocks (in addition to the plywood). This would be far more critical if the tank were glass, but it's such a simple inexpensive step I would still take it for extra precaution.
Personally... I would make 8 "legs" of cinder blocks... 4 in the front and 4 in the rear... at the 4 outter corners and the other 4 spaced evenly under the interrior... Then I would put styrofoam on top of each cinder block "leg"... then plywood... then another layer of styrofoam... then the tank...
I completely admit/confess this is "overkill" on styrofoam considering it's an acrylic tank, but again, it's such a simple/inexpensive step I would still do it personally...