How to toe-nail with 4 nails
In all the nail sizes specified in the charts, 10D (3") nails are also an option. In the building trade, that is the size pretty much used everywhere when it comes to nailing up framing in anyplace.

In answer to the 8D question, 4 nails will easily fit into the bottom of a 2X4, yes they will cross each other, but only in the plate where the split resistance will be optimal. The angle used to drive them in and the height the nails enter the stud to get a good grip into the plate, should not allow them to collide in the stud.

If the 3" nails are an issue, 3 1/4" nails are available too. I have never seen 16D nails on a shelf in any supply house either.