Display MoreThe question is of course offtopic, but I’ll answer))
MK4A uses Osprey ceramic-composite armor plates equivalent to E-SAPI Revisions A-E (not sure they stand-alone) in conjunction with soft armor panels, and side hard elements like SSAPI or ESBI. In versions MK1,2,3 side plates placement was not provided. It is also unclear whether there were different generations of plates or whether this is the same type since MK1; if so, then the level of protection is lower, at maximum, equivalent to ISAPI (2001-2009 era). I suggest this option, based on the materials that are freely available, the armorplates look low-tech and heavy nowadays. It is also unclear what type of ceramics was used (Alumina Oxide, Silicon Carbide, Boron Carbide\Diboride). I assume the first one, it is cheap, heavier than the others, less technologically advanced in production, and in terms of implementation time it is one of the first. Based on this, the limit of such plates is to hold up rifle full energy ammunition with a hardened steel core penetrator, such as 7,62x54R LPS, PS; intermediate cartridges as 5,56x45 M855, 7,62x39 BP.
Ammunition with a tungsteen core penetrator definately pass throught\clearly penetrates that plates and equivalent types\classes. Before 2010, such threats were not considered, so protection against them was not implemented in masse. The first were Granite plates according to the The Ratnik program in the Russia, meanwhile the USA supplied X-SAPI plates to all branches. X-SAPI much more technological and more expensive than Granite and can stops 7,62x51 M993 from SBR in point blank, 6B43\6B45 Ratnik cannot.
SAPI\ISAPI\ESAPI\XSAPI and probably MK4A is a special treat levels classes according to customer specifications, and customer is a government.
Or maybe MK4A plates, given British origin has European VPAM armor rating in interval PM8-PM10.
At the moment, the GOST standard for armored products is not used in Russia; BR armor classes has been in use since 2017.
Very detailed answer! Thank you for much for the info.