You would have to use flexible PCIe-extenders to connect four GPUs because if you plug them all directly into MB, they'll block ait intakes of each other.
And you can't plug them all directly because standart ATX system would accept only 7 cards (and each 5870 takes double space).
For 4 cards you would need 3 spaces in between, so a total of 7 slots would suffice. Of course, for cooling you need extenders. 1x extenders are probably better for airflow and extra flexibility, and the 1x speed is not a concern for mining.