"Old" vanity height was 32". New is the same as your kitchen counters. 36" to the top of the counter. Yes, 4" of molding would look odd. However, if you uninstall the vanity and cut off the toekick and replace that with some metal legs (think Ikea) to get to the 36", that could work well and make what's there look very current in style. Painting everything a unifying color would help to blend the old and new. Or you could use some curvy legs and overlays if that's more your style.

At our last house we wanted to update our bathrooms by getting new granite counter tops, hardware, back splash but not new vanity cabinets. We had oak vanities and added a oak strip to the top to the desired height and painted/gel stained it all to match and other than the seam where the trim board met the existing cabinets it was not noticeable. Looked more updated to be taller. I think we made ours at finished height to the top of granite 35 inches.

So to answer your question yes it can be done and does not look strange. I am pretty sure no one noticed the seam except me. Adding trim board might make the change more noticeable. Amazing what paint and stain can camouflage! Good luck with your project. .