S = square
P = 11 degree side clearance ( sometimes 10 degrees )
C = +/-.0005 IC (size of the square), +/-.001 Thickness
..... G = +/- .001 IC, +/-.005 Thickness
......U = +/- .005 IC, +/-.005 Thickness, unground periphery - not recommended for milling
4 = .500 IC dim (Inscribed circle in 1/8ths of an inch)
2 = .125 Thick ( Thickness in 1/16ths of an inch)
You are missing the last number which specifies the radius size in 64ths. #1=.0156 rad, #2=.0312 rad. etc.
This is an ANSI standard designation and is found in all the mfgs catalogs.
I believe CarbideDepot also has this on their website.