Connect 4" NPT (National Pipe Taper) to 4" BSP parallel (G4 / BSPP)

4 NPT and G4 BSP are both called 4 inch but do not interchange: NPT is a 60 degree thread form and BSP is 55 degree, so they engage a turn or two then jam without sealing. A machined NPT-to-BSP adapter is required; PTFE tape alone cannot fix the thread-form mismatch.

At 4 inch the two look close enough to force together. The 60 versus 55 degree flank angle means the threads never fully mate, so add the correct adapter rather than more tape.