Hi again Achi,this is a common misconception and there is some evidence to suggest that it might not be the case,in the context that all grades of fuel were becoming scarce.
C3 grade was needed to achieve higher boosts in various engines. "Reparatur-Anweisung 2. Nachtrag Nr. 191/345" is essentially a technical bulletin issued 14 March 1945 which explains the conversion of the 1.8ata capable DB 605 DB into the 1.98ata capable DC version. It says that C3 grade fuel was available "for use in unreduced quantity" (my translation).
"TECHNICAL REPORT NO. 145-45, MANUFACTURE OF AVIATION GASOLINE IN GERMANY, 2. Supply and Composition of Aviation Gasolines" was compiled by the allies after the German surrender. It has this to say about production of the two grades of fuel (B4 and C3)
"The relative volumes of production of the two grades cannot be accurately given, but in the last war years the major volume, perhaps two-thirds (2/3) of this total has the C-3 grade. Every effort was being made toward the end of the war to increase isoparaffin production so that C-3 volume could be increased for fighter plane use. The isoparaffin usage in that grade had already been cut to a minimum."
You are correct that the Bf109K could run on B4 fuel but by using C3 the boost could be increased to 1.98 ata and an extra 150 HP attained. In March 1945 I./JG 27, III./JG 27, III./JG 53, IV./JG 53 were ordered to keep their Bf109s but to increase the maximum boost pressure to 1,98 ata manifold pressure (OKL, Lw.-Führüngstab, Nr. 937/45 gKdos.(op) 20.03.45). It is not known how many units had converted to 1,98ata before that order came,if any. Importantly III./JG 27, III./JG 53 and IV./JG 53 were the major users of the Bf 109 K-4 in the Luftwaffe.
Cheers
Steve