What would it mean to improve from 4 pull-up reps to 20? My weightlifting book has a load estimation chart. If you know your 1 rep max, you can estimate 5 rep max, 15 rep max etc.

It says:
4 rep max - 87% of 1 rep max.
15 rep max - 60% of 1 rep max.

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Current situation of 4 pull-ups:

My theoretical 1 rep max is 61kg / 87% = 70kg. I should be able to do 15 reps with 42kg. Our gym has an ylätalja pulley which allows you to test pull-up like movement with smaller weight, and 40kg is quite close to my 15 rep max. At the exhausted end of my training session today I pulled 35kg for 15 reps. (Actually, the pulley showed 45 kg but I tested that it had 10kg bias.)

Situation with 15 pull-ups (the chart does not go to 20):

Theoretical 1 rep max is 60kg / 60% = 100kg. 30kg improvement.

On 15-rep ylätalja pulley, it means increasing the weight by 50%.

Of course these are very rough estimates and require that you actually train both 15 rep series and 1 rep max, but they give you some idea what sea change it is to improve from 4 pull-ups to 20 pull-ups.