Aside from the dramatic reduction in cross-sectional area (almost half), you'll create a fairly severe lint-catcher. If you can enlarge the opening, I would.

You can probably convert to a rectangular vent that equals the cross-sectional area of a 4" duct. You may have to get creative. The area of a 4" circle is about 12.5 inches square, so 3 by 5 would do, for example.

I would not reduce the area (as with a 3" round vent) as you'll increase backpressure and create a lint trap.