Looping a bassline over an odd number of beats phases it against a 4-beat drum pattern

When a bassline loops over a length that does not divide evenly into the drum pattern’s length, the two cycles fall out of and back into alignment over time instead of repeating identically every bar. The tutorial loops its bassline for five beats against a four-beat drum pattern: because 5 and 4 share no common factor below 20, the bass lands in a different position relative to the drums each bar and only realigns after their least-common-multiple span. This introduces long-form variation from static material — the combination never settles into a single repetitive loop even though each part is itself a fixed loop.

Examples

Loop a 5-beat bassline against a 4-beat drum pattern; the pair only exactly repeats every 20 beats, so bars 1-5 each present the bass in a new relationship to the kick. Contrast a 4-beat bass loop, which repeats identically every bar.

Assessment

Why does a 5-beat bassline loop create evolving variation against a 4-beat drum pattern where a 4-beat loop does not? After how many beats does the 5-against-4 combination exactly repeat?