I sweat heavily when hiking, particularly when going up hill. So keep this in mind when you read my comments. They won't apply to you if you are not a heavy sweater.

"Have you ever hiked for 4 hours with a pack (at least partly up hill) in pouring down rain?" Yes. This is the typical Pacific Northwest spring and fall hike for me. Gain a couple of thousand feet or more and camp in an area that is high, cool/cold and windy. The rain may have turned to snow by the time I reach camp.

"Did you stay dry (10,20,30....100%?)? Never (0%), even if it isn't raining.

"What were you wearing?" I keep moving and wear as little as I can to stay warm. Everything will be soaking wet by the time I get to camp and I will have to strip naked and redress. I gave up trying to keep dry decades ago and now only try to keep warm. I wear all synthetics because they retain less water and/or dry more quickly than cotton, down or wool. I've even experimented with closed cell foam clothing (e.g. a float coat) because they stay warm when wet.

"Do you think most of the water was from the outside (rain) or inside (sweat)? The sweat alone soaks me 100%, even without rain. Every single thing I am wearing will be wringing wet by the time I reach camp if I'm going up switchbacks with a pack. If it is raining then some of the sweat will be replaced by cold rain but the clothing can only hold so much. I can't really get any wetter.

I hike with a friend who keeps dry during everything I have described. Like I said at the beginning, my experience may not apply to you.