The easiest way to check if a cell contains a specific text string is, in my opinion, the IF and COUNTIF function combined. The COUNTIF function allows you to count how many times a text string exists in a cell range.
The asterisk characters let you perform a wildcard match meaning that it matches any sequence of characters. Adding a beginning and ending asterisk to a text string allows you to check if a cell value contains a specific text string.
=IF(COUNTIF(B4, "*"&C4&"*"), TRUE, FALSE)
You don't have to add asterisks manually to your cell, you can easily build a formula that adds the asterisks automatically, demonstrated in cell D4. The ampersand & character lets you append the asterisks to the text string you want to use.
Note, if a text string is found twice in the same cell the COUNTIF function only returns 1. It counts cells not text strings.
The SEARCH function returns the position of the character at which a specific text string is found. Luckily the IF function accepts any number as TRUE except 0 (zero), however, the SEARCH function returns an error #VALUE! if it can't find the text string.
To avoid the error value I use the ISNUMBER function that returns TRUE if a number and FALSE if anything else, also formula errors.