The words forth, fourth sound the same but have different meanings and spellings. Why do forth, fourth sound the same even though they are completely different words?
The answer is simple: forth, fourth are homophones of the English language.
Forward in time, place, or order; onward: from this time forth.
Out into view: A stranger came forth from the crowd; put my ideas forth.
Obsolete Away from a specified place; abroad.
Archaic Out of; forth from.
The ordinal number matching the number four in a series.
One of four equal parts.
Music A tone four degrees above or below a given tone in a diatonic scale.
Music The interval between two such tones.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.