I can't recommend a brand, but I can recommend a formula. This is from David Johnson, who carved under Ozark Mountain Briar.
For adequate buffing you need a surface speed of somewhere between 2000 and 4500 fpm. To calculate fpm, multiply the circumference of the buff (diameter x pi) by the rpm, then divide by 12.
In this instance, your 4" buffs have a circumference of 12.6 inches (4 x 3.1416). 12.6" circumference x 1950 rpm = 24570 sipm (surface inches per minute) divided by 12 = 2047.5 sfpm (surface feet per minute) Which should be adequate for your buffing needs. This is just a guide though so try both higher and lower speeds to see what works best for you. I buff with brown tripoli and white diamond at 1800 rpm with 8 inch buffs which gives me the best result.
White diamond compound should be available from most online woodworking sites. I bought mine from Craft Supplies USA Buffing Compound - Woodturners Catalog - Woodworking tools and supplies specializing in woodturning.