Some 17.5 motors like more than 4.0 and some like less. Eg. I ran my D3 at 4.3ish, by my D3.5 is 3.6ish. If a 39T (48P) pinion won't fit with the 84T spur, then go down in teeth until something does fit. Like you, I also have not seen any info on the maximum sizes of gears that'll fit on the T4.

EDIT: I miss-read your OP as I was understanding that you already had pinions. Now I see that you're ordering pinions. My above statement is therefore essentially useless.