US Congress Conference Report which has not been voted on or passed yet, refers to the final version of the bill - does not allow a deduction for prepayment of 2018 state income taxes.
https://rules.house.gov/conference-report/hr-1
In addition prior revenue rulings 71-190 and 82-208 also preclude taking a 2017 deduction for payments made in 2017 to be applied to a tax liability in 2018 — notably, a year that has not even arrived at the time of payment, let alone produced any income, deductions, credits, or other items to support a tax calculation. Any payment might be in the form of an estimated tax — to be credited against the 2018 tax liability when it is determined.
What does this mean for individual taxpayers? Payments for the 2018 state & local taxes prior to December 31, 2017 will not be able to be deducted on your 2017 personal tax return as an itemized deduction.
This does not preclude individuals from paying their 2017 State estimate #4 prior to December 31, 2017 and including this as an itemized deduction
The conference agreement also provides that, in the case of an amount paid in a taxable year beginning before January 1, 2018, with respect to a State or local income tax imposed for a taxable year beginning after December 31, 2017, the payment shall be treated as paid on the last day of the taxable year for which such tax is so imposed for purposes of applying the provision limiting the dollar amount of the deduction.