UKC

.....When I could only do 3 yesterday, am I 25% stronger?

Your strength (-endurance) was 25% less yesterday than today's 33% more. Or something like that.

"Less weak" would be more appropriate than "stronger".

no. you might have only needed that 1 percent extra to get you over that next hurdle. you never fail at the very bottom of a chin up, and once you get past a certain point in the motion it gets easier, so it might be that 1 per cent strength gain (or more likely, that few grams less body weight due to when you last ate/drank/pissed) gets you past the point where the motion becomes easier, where you had enough strength to complete that part yesterday, but you didn't have the strength to get to that part.

"Less weak"... mioW!   I know a climber who, at the time when they had climbed 8b, said they could only do 6 chin-ups... admittedly their finger strength and endurance was utterly ridiculous

Only if you are as close to doing one more pull-up today as you were yesterday. So the only way to find out is attempt to do 5 tomorrow.

33% stronger. 

As above tomorrow you should be able to do 5.3333.

I'm not sure if your strength has gone up, because you're only lifting the same weight. Power endurance(?) has gone up by 1/3 though. Nice one!

Front lever by end of the week at this rate!

If you do 4 one armers you'll save 50% of your strength which could be used to do 4 more with the other arm thus seeing a 100% improvement.

Yes absolutely.  You should start getting on E1.25 routes.

If you want a sensible(ish) answer use this https://www.exrx.net/Calculators/OneRepMax for an approximation of your 1RM. Use your bodyweight.  Ideas of increase in "strength" in low rep ranges are usually pretty well indicated by projected one rep maxes.

It's more like a 4% increase,
 

How much time between each pull up?

An amazingly sensible bunch of replies to a very silly post! Thanks all, as suspected the answer is 'No'. I will keep doing them anyway.

If 4 is really your max and you want to improve quickly.... don't do 4s or 3s every day... do 2s, with an hour between sets, MANY times in the day. Revisit those numbers each week. It will go up by 1 or 2 reps per set per week at least. If you stick to it I guarantee you will be cranking consistent sets of ten within 6 weeks.

25% stronger would mean doing those same 3 pull ups but with 25% more weight. That is 25% of your body weight. So if you weigh 70kg then you'd be adding 17.5kg. That'll be considerably harder than simply adding a rep or even half a dozen reps to your set.

However adding a rep, especially in the low rep range, is still a good improvement. Keep it up. You could start adding weight when you get in the 10-12 rep range.

I'm coming at this a different way :- 

If I just get 25% fatter, but can still do the 3 pull up reps , will I still be 25% stronger?

will my wife believe me that I have been in the gym, or suspect I have been in the pub ? 

Here's a fun table : https://strengthlevel.com/strength-standards/pull-ups/kg

If you click on the one-rep max checkbox and it will show that below the reps for bodyweight only.

You'll always be rounding down when it comes to reps. It could be that you had the strength to do 3.9 pull-ups, which meant you only managed 3 full pull-ups. So 3.9 to 4 is very little increase. It depends how far off 4 you were the first time. Tiredness/fatigue will also play a part. I can often manage more reps after a short break from training in order to recover. 

Depends on how warmed up you are as well.