Also keep in mind that one of 4ed best features is that monsters dont follow any standard rules. As the DM you just figure out what the monster should be able to do and choose a mechanic that simulates that. It doesnt matter that the players, or other monster, can't do the same thing.
So it sounds like you want your constructs (I figure they sound like Brutes) to have a powers that goes something like this....
Unstoppable Plodding
At-Will
Standard Action
Attack: Level +1 vs. Fortitude
Hit: (Damage determined as per DMG page 185). Target knocked prone and the construct shifts into the targets square.
Beneath my Notice
At will but see requirement
Requires prone target
Standard Action
Attack: lvl +3 vs. AC
Hit: (Damage determined as per DMG page 185).
Effect: The construct moves it speed.
By setting the powers up this way the construct cant do both attacks in one turn, because that might be to powerful (two attacks in one turn). But that can easily be changed by making Beneath my Notice a move action.
Consider giving the PC lying prone beneath the construct a substanstial attack bonus (is should more than equalize the -2 penalty for being prone) for attacking the "soft" underbelly. This will make it a risk/reward decision to roll out and stand up or stay down, make an attack with a bonus, but risk being trampled.