The P4 was a stop gap measure by Walther.
The German police were looking for a better pistol after the Olympic terrorist incident.
Walther was developing what would be the Walther P88 but it wasn't ready yet, but the police wanted a gun NOW.
So Walther did an upgrade to the P-38/P1 to give the police a better pistol until the P88 was ready.
The safety had a spring installed that turned it into a de-cocker. The function was the same, just when the safety was applied, it snapped back up under spring pressure.
The troublesome slide top cover of the P-38 was deleted along with the loaded chamber indicator.
The firing pin system was redesigned to prevent the hammer from hitting it when the safety was applied and a hole was drilled in the hammer to further prevent the hammer from contacting the firing pin when the safety was applied.
When the P-38/P1 safety was applied the firing pin was locked and the hammer de-cocked to drop on the locked firing pin.
The P4 lifts the firing pin out of the path of the hammer and locks it. When the hammer drops, it can't contact the firing pin at all.
The barrel was shortened.
The frame and internal parts remained the same as the P-38/P1.
The P-38/P1 rear sight interlocks with the top cover and they hold each other in place.
The P4 rear sight is attached to the slide with a screw. Contrary to popular belief, the P4 rear sight is NOT windage adjustable.
The P4 was primarily intended for German police sales, but it was sold commercially here in the US for a short time.
I wasn't aware that the P4 was ever made by Manurhin. I'd guess they were made during the period when the Walther licensing had been discontinued and Manurhin made Walther guns under their own name.