Replace cell in Column A with value in Column B if contains similar character string

Hello all! I am in need of a formula and am having a problem finding a solution.

I have values in column A and in column B.
Column A contains values with 4 to 8 characters, and Column A has 300 cells.
IE: 1234, 1259T, G8797TB
Column B contains a list of values that lead with the same values, but then has additional information, and the column has 100 cells.
IE 1234_HELP_ME PLEASE, 1259T_SEEMS_SO EASY, G8797TB_AM_I DUMB

I need to figure out a formula that will check if Column A has "1234" then replace with value in Column B "1234_HELP_ME PLEASE".

I can resolve this by doing find and replace 100 times, but am sure there is a better way. Also the data in each Column are not sorted alphabetically if that matters. I am using Office 2010.

Thank you so much for the help!

Thanks! I have been learning about excel, and am excited to learn more on this forum!

In the few formulas I have used, I have always used a helper column.

Excel example of request.xlsx

I believe I have attached the spreadsheet.
Column A has the values that I want to keep. These values would be used to replace the values in Column B.
Column B has the values that I want to change, with the corresponding values in Column A.
Column C is what the final product would look like, after the formula or perhaps a find and replace macro.

Let me know what you think, and thanks for the help!