Calculated columns with letters

Occasional Visitor
Hi all I'm looking for some help.
I have a list with exam questions.
Each question has 4 choices. They are a,b,c,d
The exam is run through power apps.
However I need to calculate a final exam score.
The correct answer is given a score of 4 the rest are 0.
I have no idea how to weight the alphabetical values with a numeric one.

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