Watch the Subbin' online guitar lesson by Brad Carlton from Chord Studies: Evolving Blues Vol. 1
Measure 4 gives us a "sub" chord - which is a "substitute" chord. Here we are using a b5 sub of the A7 chord, the Eb7b5. When you get into substituting one chord for another, there are rules, and guidelines that help you pick a chord that sounds good, but in the end it is indeed what sounds "good" to you that should dictate which ones you use.