The venting pipe for my kitchen sink runs horizontally for a run of about 5 feet before it turns and runs up the wall and out. My problem is that the pipe runs about 4 inches above the level of my counter top. I have a new window to install there and the rough opening framing will end right in the pipe run. Can I drop this vent run down,( about 4-5 inches) for about two feet and then use a 90 degree to return it to its original run, connecting up again with the pipe going up the wall.

Why don't u re-route your drain if u are on a subfloor and run up vertically along the king stud and dirty arm over to kitchen sink with your trap arm.
U may if your code allows u to do so. If so use medium sweep 90 for vertical to horizontal with vent, then use a long turn 90 to go up the wall. If this is exterior wall, run horizontal vent exposed under sink and run it back into wall and up.

States have different codes, but most codes require a vent to run vertical to 6" above flood level rim of fixture it is venting then may run horizontal until it turns to the vertical again.

Yes, with structural issues where he has to install a window right where existing vent is, upc and cpc allows vents to run horizontal and slope it @ 1/4" per foot, or like I said run the vent along the king stud vertically and dirty arm with trap arm to the existing sink which is more work.