Correct spelling, explanation: 4th is an ordinal number and its full form is spelled fourth. It appears between third and fifth. It’s one of the regular ordinal numbers that are formed by adding -th suffix to their roots. How do we know that it’s regular? Because it’s not one, two or three. The first three numbers are formed into ordinal numbers differently (one changes to first, two changes to second, and three changes to third). The abbreviation is -th, not -rth. The letter -r is a part of the number. That’s why you shouldn’t spell it as 4rth.

Definition of 4th:
numeral, coming next after third in position
The 4th of July is a national holiday in the United States, celebrating the country’s independence.
She finished in 4th place in the race, just missing out on the podium.

Collocations with 4th:
Some most commonly used collocations include: 4th quarter, 4th grade
The game was tied going into the 4th quarter, but the home team pulled ahead and won in the end.
My little brother is in 4th grade this year.

Incorrect spelling, explanation: that different ordinal numbers have different suffixes: -th, -st, -nd, or -rd. It all depends on what number is at their end. As you can see, there’s no place for -rth in this group of abbreviations. This mistake may occur because the full form of 4th is fourth, but the -r letter is a part of the number. You can’t spell this word as 4rth, because that would be an error.

Incorrect spelling

Incorrect spelling

Incorrect spelling