These numbers are the way the thickness of hardwood lumber is measured. They refer to the number of quarters of an inch (i.e. 4/4 is 4 quarters of an inch or 1”). The measurement references the rough (unsurfaced) thickness of the lumber.
Once the lumber is surfaced you will loose from 1/8-3/16 in thickness. For reference here are the typical surfaced thicknesses:
4/4 – Surfaces to 13/16” or better
8/4 – 1-13/16” or better
10/4 - 2-5/16” or better