Ok Awasoda .... just based on the 4th A.D. ..... I've found pics of everything from M4A3 (75) through M4A3 (76) E8 with a smattering of M4A1's thrown in just to confuse the issue. Toss in a few Jumbo's and you see where the problems begin.
The 4th ... like all divisions was composed of many different units. In this case your Grandpa probably belonged to the 8th, 35th or 37th Tank Battalions. Unfortunately, trying to pin down which Sherman or what his markings were would require a bit more research on your part.
As far as camo goes .... O.D. seems to be the most common in the pics that I've looked at with a little whitewash thrown in for good measure.
Now an interesting build you may want to consider is Colonel Abrams M4A3 E8 who was probably one of the more famous members of the 4th. Or another option would be the Jumbo with the chalk markings "First in Bastogne".
I'm sure whatever you decide to do, your Grandpa would appreciate the effort.
Happy Modelling and God Bless
Robert