Yes. There isn't a 4X card in existence that cannot do 2X as well.
The only combination you have to watch out for is 2X/4X/8X cards in 1X-only motherboards, and conversely 1X cards in 2X/4X/8X motherboards. They are the only combination that can cause damage, as the AGP1X specification uses a higher voltage and will damage either the board or card if used incorrectly.