Modern Chess openings - Reading the notation

Hey all! I've recently purchased the book Modern Chess openings, and am a bit confused about the notation. 

It looks a little like this:

1 P-K 4. KT -K B 3 : 2 P-K 5. KT-Q4: 3P-QB4 KT-KT3: 4 P-Q4. PQ-3

           1             2           3           4          5

5 P-B4 .....................................................PXP
PXP                                                KPXP

6 BPXP                                           KTQB3 (f) 
KT-B3                                             KTQB3(I)

I am clear on the meanings behind the letter, KT-KB3 meaning Knight to Kings Bishop 3, but i get really confused as soon as the line starts to run.

When we approach 5. It seems like there are 2 variants, PXP or Pawn to Bishop 4 for white, but what do the moves to the right mean? Are they the correct responses from blacks perspective? So IF white plays P-B4, does black do PXP? Or am i completely lost here?

That's descriptive notation and it stopped being used since 1981. 

I think this is it, if I read that correctly. Not used to it to be honest. Yes I believe those are alternative lines.