Using Framed function on a list with MapAt

Use MapAt to apply the function Framed on all the letter elements in the following list:


While solving the above problem, the following MapAt identifies the third and fourth strings on each element on which Framed function is to be applied.

MapAt [f,{{1,2,"A","B"},{3,4,"C","D"},{5,6,"E","F"}},{All,{3,4}}]  

Now, I intend to use Framed function. I tried the following way, which is not correct as the variable x understandably is instead given the value of "Framed" string.

f[x_]:= Framed 

Wondering how to apply Framed function in this context.