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For the uninitiated: the G-4 visa is for a) government employees on official business of their foreign government or b) employees of international organizations representing the organization; these visa holders do not enjoy diplomatic status.

The short answer is yes, you should have no problem using the diplomatic lines because you are on official business.

IAD, being the nations capital, regularly receives travelers presenting uncommon visas and all but newbie staff in all lines will have experience processing many of these types of uncommon visas (as they will not be uncommon at IAD). On the other hand, at smaller ports of entry many of the staff in the "regular" lines will not have experience and might well need to call in a supervisor or refer the pax to additional inspection for admittance.