Depends on what you are trying to do I guess.. Normally you wouldn't use it in an AG brew, as you can control that a bit with your mash temp and yeast choice. However, people do use it when they need to adjust their recipe a bit more than they can control. Whats the recipe?
11.25 lb American 2 row pale 4 oz 120°L American crystal 1oz northern brewers (bittering) .5oz cascade (flavor) .75oz centennial (flavor) 1oz cascade (aroma) 1tsp Irish moss wyeast 1187 1oz cascade (dry hop)
What does the extract recipe look like? I can't see why you'd want maltodextrin in the AG, and especially the extract.
I've never brewed from extract, but here goes. Take out the 2 row pale and add: 4 lbs Alexander pale malt syrup 3.33lbs muntons extra light DME 4oz maltodextrin
It looks like you're tring to replace C-120 with maltodextrin, but maltodextrin won't contribute any flavor or color (and probably isn't needed here for it's true purpose, which is adding unfermentable gravity). Why not just steep C-120?
4 oz of maltodextrin isn't going to make or break this recipe either way. Your options are: A) Use it. B) Don't use it and do nothing to compensate. C) Don't use it and increase your mash temp slightly to compensate. Personally I don't see a good reason to use it, but honestly any of the above options are going to work fine for you, it's not really going to make much of a difference.
Maltodextrin is used in order to increase the amount of nonfermentables in the wort. This is most useful in extract brewing, where you're beginning with a kind of "one size fits all" wort and you want something with a thicker mouthfeel. In AG brewing, you can increase the nonfermentables by using a high mash temp and/or by using carapils or similar. So... if it was my beer, I'd probably try to get the same effect by mashing high, around 156F. And if that didn't give me the results I wanted, next time I'd add some carapils. But you can use the maltodextrin if you'd rather.
