Does a roth withdrawal from a 401k show as income when calculating the payment to SS for part B?

Since you had the Roth 5 years and are over age 59 1/2 the withdrawal is tax free and not considered income for calculating your premium for Medicare. 

@Ry03 I think you are asking about IRMAA surcharges.

"MAGI" is calculated by taking your AGI (Line 11) and adding tax-exempt interest (line 2a).  That is it!

Income reported in box 4a, which would include Roth distributions, are NOT added back for IRMAA Medicare premium purposes. 

thank you!!!