Why did the B-52 never have a JT4A-powered version?
The military designation of the proposed JT4A is J75 (not to be confused with J57).
It was proposed for the G-variant of the B-52, but it would have slowed down the production:

The B-52G was proposed to extend the B-52's service life during delays in the B-58 Hustler program. At first, a radical redesign was envisioned with a completely new wing and Pratt &amp; Whitney J75 engines. This was rejected to avoid slowdowns in production, although changes were implemented. The most significant of these was the brand new &quot;wet&quot; wing (...)
—livingwarbirds.com

Even a simple re-engine (as opposed to re-wing and re-engine) takes a lot of resources and time. The new engines are about 10% heavier, and they would need new nacelles, aerodynamic and flutter testing, etc. So, it was considered, but it wasn't the right time – fast production was needed for its role in the Cold War; 193 B-52Gs were delivered 1959–1961, making it the most produced variant.