Incorrect spelling, explanation: there is no such form as 4rd, because the suffix -rd is used only with number 3, for example, 3rd, 43rd, 183rd. Number four uses the regular suffix -th – 4th, just like most numbers, for example, 8th, 7th, or 10th. The full-word form is fourth, so th also translates to pronunciation. 4th is therefore the correct form.

Correct spelling, explanation: this form is the correct one because the full-word form of 4th is fourth, so -th is the suffix for this ordinal number. -th is a common suffix, but there are also irregular ones: -st, -nd, and -rd. The last one is used only for the number 3, so 3rd (third). Number 4 has a regular suffix, so the correct form is 4th, not 4rd.

Definition of 4th:
numeral, the ordinal number between 3rd and 5th
I did pretty well in the biology contest, I was 4th out of 25.
Nicky is my 4th dog and he is the most lively one.

Incorrect spelling

Incorrect spelling