4mm is perfectly safe on a 77, work out the static compression ratio and you will see why. A Diana 52 has a 6mm long TP with seal length and in unrestricted mode has 4mm dia TP...no different.

peak power comes under 4mm but not by much, i often see gains over over 1fpe going from 3 to the sweet spot with the same spring and preload, this then allows the spring to be backed off etc.

If using an O ring head the TP needs to be a tad smaller than on a synthetic seal, this is to raise the compression, O ring heads are more efficient and make power easier, so a smaller TP can be used and you get the same results.