I agree with Ralph's advice.

When the fish is headed down deep (or to fast current, for that matter), you need to turn that fish before they get the upper hand.

You said that you usually use 4x tippet for nymphs. Do a little test and rig up your rod, using 4x tippet. Tie the tippet to something solid near the ground.
Back up from the 'anchored' tippet about 20-30ft and bend the rod as though you had a fish on the line. Use only steady, even pressure.
Keep a good angle on the rod, using the entire length of the rod to 'fight the fish'. See how much leverage you can put on your 'fish' with that 4x tippet, again only using steady, even pressure. You may be surprised by how much pressure you can put on a 'fish'.

A guide friend of mine showed me this some years back. We were using 5x tipped. He just stood in the stream and held the tippet wrapped around a couple fingers.

I was amazed by how much force I could put on the line without breaking it.

Hope my 2 cents helps.

Brian