5.35Bar 4: ii–V into IV

Instead of staying C7 in bar 4:

Gm7 → C7

Why?

Because bar 5 is F7.

In F:

Gm7 = ii

C7 = V

F7 = I-like destination within the blues.

So:

Gm7-C7 → F7

is a ii–V preparing IV.