i have a idea =)
your 4 textures:
RGBA, RGB, RGBA, RGB
size:
512x512

combine
RGBA + RGBA
RGB + RGB

texture 1 with 2:
1024x512
texture 3 with 4:
1024x512

and in shader scale in tex2D, X with 0.5

and if you want use mip's

all 4 texture in one:
1024x1024

and scale in tex2D Y with 0.5 too

(i think mips need same x and y size. right?)

edit:

ent texture 1 = 512x512 your sub surface map with the UV
ent texture 2 = texture 1+2 1024x512
ent texture 3 = texture 3+4 1024x512
ent texture 4 = free

positive:
its work ^^
negativ:
reading biger texture / need more performance

i dont know.. what is faster? read 2 texture in 512x512 or one texture in 1024x512?...

edit2:

or:

ent texture 1 = 512x512 model color texture
ent texture 2 = 1024x512 texture 2+3
ent texture 3 = 1024x512 texture 4+5
ent texture 4 = free