forth / fourth are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
forth: (noun) a river in southern Scotland that flows eastward to the Firth of Forth. (adverb) forward in time or order or degree. (adverb) out into view.
fourth: (noun) following the third position; number four in a countable series. (noun) the musical interval between one note and another four notes away from it. (adjective) coming next after the third and just before the fifth in position or time or degree or magnitude.
forth: f·AO·r·th
fourth: f·AO·r·th
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)