If a multi-intercoastal block is performed bilaterally you report 64421 with the 50 modifier with one unit.
You mentioned intercoastal but then mentioned T4-7 so I assumed you meant mult-intercoastal meaning multiple injections were performed to target T4-T7 on the right and left side.
Here is AMA CPT Assistant article that mentions the codes in question
August 2010 page 12
Surgery: Nervous System, 64420, 64421 (Q&A)
Question: If a single injection is performed for a thoracic intercostal nerve block, the effect of which is to block multiple intercostals nerves, is it appropriate to report CPT code 64421, Injection, anesthetic agent; intercostal nerves, multiple, regional block, or is the provider limited to reporting CPT code 64420, Injection, anesthetic agent; intercostal nerve, single?
Answer: "Single" injection refers to a single nerve block, so code 64420 should be reported. However, if only one needle is placed into the skin but manipulated up and down to inject into more than one nerve, it may be possible to report multiple blocks (code 64421). However, from a clinical perspective, blocking multiple intercostal nerves with a single injection is hard to do in most settings.