The P4 is an interesting variation that never really took off.
To those not familiar with the P4 while they look similar to the P1 with a slightly shorter barrel they have several very different features, this is all from memory as I'm too lazy to dig my P4 out right now lol...
The P4's big upgrade is that it can't be drop fired or fire with a worn firing pin during decock like the P38, the P4 has a system that raises the firing pin into battery as the trigger is pulled, there's also a hole cut below the hammer face that the firing pin rests in, btw those features carried over to the P5.
Another big one is that the safety lever is a spring loaded decocker so it doesn't remain down, no need for it to because of the trigger safety.
The area just ahead of the rear sight is different but can't remember how or why, as a side note the factory short barrel guns with the front sight on the slide front were made from P4's IIRC.
On a side note I've handled a few P88's thru the years but never fealt they were worth the huge premium they commanded compared to the P7M13 in the same range back in the day, well perhaps the Beretta 92 was as well but at much less, also was not crazy about the looks (striking resemblance to the Zastava CZ99).
Just Random thoughts,
Btw last time I handled one they were fetching $1k used when S&W 3rd gens were $350.
My guess is their high price and chunky look led to poor sales and their current rarity in the market, not sure but think clean P88s are about double today.