Add a row with +1 to the previous row cell value

probably a simple thing but ive been out of excel for a long time and cant remember

I have an excel sheet with row ids that are in the format CXXXX where X is the number increment. If my previous row has the id of C7456 i need it so that when a cell is NOT empty it will show the number for the next row.

Below is the formula i have but it results in an error;

=IF(D185="","","C"& TEXT(RIGHT(A185,4)+1,"0000")