Assuming that you did not make any after-tax contributions to your 401(k), your entire 401(k) distribution adds to your AGI and is treated as ordinary income unless it is rolled over to another retirement account. This income is included in the MAGI used for determining eligibility for health insurance Premium Tax Credits (and any other any type of modified AGI, as far as I know).
The distribution from the 401(k) would only be excluded from the MAGI for determining your eligibility for a Roth IRA contribution if the 401(k) distribution was rolled over, either to a another pre-tax retirement account (which would keep it from being part of your AGI in the first place) or to a Roth IRA (in which case it's subtracted from your AGI in determining your MAGI for this purpose).
4. Enter any traditional IRA deduction from Form 1040, line 32; Form 1040A, line 17; or Form 1040NR, line 32................................... does this include 401-K from your job?
Elective deferrals *to* your traditional 401(k) from your paycheck are not included in your AGI and therefore do not appear in your MAGI. They are excluded from the amount reported in box 1 of your W-2. They are not reported anywhere on your tax return as a deduction.
Elective deferrals to a Designated Roth Account in your 401(k) (a Roth 401(k)) do appear in your AGI and MAGI since they are not excluded from the amount in box 1 of your W-2.
