A hypothetical organism is tetraploid and isogametous. To reproduce, four haploid gametes must meet and fuse into a tetraploid zygote. Any four individuals are sexually compatible, barring self-incompatibility or mating types.
What selection pressures would favor this method of reproduction over the fusion of a pair of ovum and sperm?
EDIT: near as I can tell, this would be a defense against recessive traits, frequent population bottlenecks and frequent mutations. The downside would be a slower overall rate of evolution, I think.