Watch this video on take down of the 4506. It's not much if any different than yours. Sometimes you have to take a small screwdriver and gently push that one particular arm down as some seem to be especially aggravating to push down with the thumb nail. I have one like that. Just a gently push is all that's needed if your fingernail won't work. That smallest arm(or pin) is not going down all the way. I have had trouble with my 4506 occasionally with it.

Watch especially from about 3:00 on through the end of the video. It's a good video but a bit out of sink or slow with the video speed.

YouTube - Smith & Wesson Mod 4506 advanced takedown