"mus_f_part4 - Undertale" is a high quality rip of "mus_f_part4" from Undertale.
Joke[]
While "Your Best Nightmare" is divided into multiple audio files that include "mus_f_part1", "mus_f_part2", and "mus_f_part3" (which is actually mus_f_repeat & mus_f_repeat2), there isn't actually a "mus_f_part4".
This is a portion of "Your Best Nightmare" edited, with various Undertale sound effects, to sound like "Rubber Band".
The track title references its use in the "penis music" meme.