I go by the following rule of thumb:

If I use 1 layer of tape to hone, I strop with no tape and let the middle of the strop sag no more than a half inch. For a 3 layer bevel I let it sag no more than 3/4 inch in the middle.

For your 4 layer bevel, I think 3/4 inch of sag is still plenty. This is just my opinion because I never used 4 layers but thanks for the idea.....

Jerry
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