If you told Turbotax during the installment application (assuming you filed a Form 9465 through TurboTax), then a Form 1040-V voucher should have printed along with your return for that amount. You should send your check for the $4,000 along with Form 1040-V to the IRS. It won't need to be received before April 18th, as long as its postmarked by 4/18, you will not be charged interest/penalties on that amount - though, you still will accumulate on the rest of your unpaid balance.
Also, you don't have to use the check/voucher method. You can make the same payment online directly at the IRS website if you wish and you can just ignore the printed voucher -
https://www.irs.gov/payments
