Your basis the amount reported on line 14 of your last-filed Form 8606 prior to 2017. That would be $4,000 on line 14 of your 2012 Form 8606 if you never made any other nondeductible traditional IRA contributions.
It doesn't matter what account received your nondeductible traditional IRA contributions. Your basis in nondeductible traditional IRA contributions applies in aggregate to your traditional IRAs in aggregate.
