What formula would you enter in C4 that combines A4 and B4 to
get a result like in C2?
=A4&&B4
=A4&" "&B4
="A4"&""&"34"
=A4&""&B4

Since the question is missing the contents of A4, B4, and C2, see below the explanations for two possible cases.

Explanation:

The command that is used in excel worksheets to combine the content of two (or more) cells into other cell is &.

The content on the resulting cell is will be a text, independent on whether the starting cells had text or numbers.

If you do not want to add any character or blank space in between the content of the starging cells, just use &.

The procedure is:

Select the cell where you want to put the combined data: C2 in your case.Type = or + and select the first cell you want to combine: A4 in  your case.Type &Select the other cell that you want to combine: B4 in your casePress enter.Done.

Thus, if you do not want to add any character, the answer would be:

=A4&B4. That is not a choice. But notice that =A4&""&B4 yields the same result.

If you want to add some characters or blank space in between the content of the starging cells,  use &" whatever you want to add in between"&

The procedure is:

Select the cell where you want to put the combined data: C2 in your case.Type = or + and select the first cell you want to combine: A4 in  your case.Type &Type "Type whatever you want to add in betwenType "Type &Select the other cell that you want to combine: B4 in your casePress enter.Done.

If you you want to add a blank space the answer would be:

=A4&" "&B4.