90 grn Muzzys in 4 blade will take an elk down. My concern would be if you' re shooting a head that light, then your total arrow weight must be fairly light as well. In Idaho the minimum total arrow weight which includes broadhead must be 400grn. We' ve taken a few bulls with 100grn 3 blade Muzzys, but now use 125grn 3 blade and 130 grn 4 blade. It offers a little more cutting surface as well as weight. Elk are tough as you well know, a little heavier arrow setup is better than to light a setup for those marginal shots where penetration can be hindered.
Good Luck. elknut1