He wouldn't have to guard Tim Duncan. Amare Stoudemire or Boris Diaw would. The number of teams with two bigs who are a) larger than Grant and b) capable of scoring on the low block isn't nearly as large as you'd think. Plus, in Phoenix's system, you'd just take advantage of the mis-match at the other end.
I'm also not talking about Grant playing 40 minutes at the 4. But he will play there at times, and he should be fine.
Even when Grant does spend some time at the 4, I expect him to jack up quite a few more 3pt attempts this season than he has any other season in his career. Will be interesting to see how that works out given that Grant has never been known as a perimeter shooter. Though could make it all the more advantageous for him to spend time at the 4.