VBA code for text to number

Hello all
I'm currently working on an excising excel document, which we use to keep track of client progress, i would like it to change so anytime someone inputs "na" into a cell to would convert it to a 4 within a certain range, so when we use it in several formulas it would calculate appropriately, does anyone know how i can do this?

Thanks Mike that worked perfectly

@ Mellie_d1

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Thanks.

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ok so now that i have that fix, i was wondering if there was something i could add to this code to also make the font red of the ones that get changed to 4 from na

thank you for your quick response but sometimes they input "4" and sometimes they input "na" which equals to 4 in the end and i need to distinguish the two, is there a way to do this?