I`d be interested to hear where the source for contention is on this, its pretty well established.
The only times where the 801 was on B4, were versions 801 C or earlier, and later on as an emergency measure when there wasnt
any C3 about, in operations many engines were de-rated to use whatever was on hand (i.e lowering the boost). However as far
as factory specs go, the 801 D onwards was definetly not specified for B4.
Its pretty easy to differentiate BMW 801 A,B,C to D versions as the A,B,C on B4 are all limited to 1.32ata manifold pressure. These were typically bomber engines.
C3 came in with the D version: (this is in English as its from the British Intelligence file on the 801)
As far as 801 unreliability, there are many aspects to that, and there was a lot of work reworking the exhaust pipes etc and improving the cooling
as was the case for all air-cooled engines. However, most of the problems were metallurgical in nature, and indeed the 801 was
deeply unreliable until somewhere around late 1942. There isnt really a "cut off date" for that, the issues were always ongoing to various degrees.