But-

but- is a chemical prefix. It was extracted from "butyric acid". It serves as the number 4 when counting axial atoms in forming names of organic compounds. As an example, butane is an alkane with four carbons.

Carbon-based
-ane (alkane) · -ene (alkene) · -ine (unsaturated hydrocarbon) · -yne (alkyne) · alk- (saturated hydrocarbon) · ar- (aromatic)
Oxygen-based
Nitrogen-based
-ine (alkaloid) · aza- (N replaces C)
Sulfur-based
thio- (S replaces O)
Counting axial atoms
meth- (1)  · eth- (2)  · prop- (3)  · but- (4)  · (and the rest are ordinary Greek/Latin numbers)
Other
-ase (enzyme)  · -yl (radical) · nor- (no residues)