Upon the conviction of a person on the second offense only, a judge may sentence such person to participate in a court approved inpatient alcohol or drug treatment program pursuant to subdivision (a)(4). For the third conviction, there shall be imposed a fine of not less than one thousand one hundred dollars ($1,100) nor more than ten thousand dollars ($10,000), and the person or persons shall be confined in the county jail or workhouse for not less than one hundred twenty (120) days nor more than eleven (11) months and twenty-nine (29) days, and the court shall prohibit such convicted person or persons from driving a vehicle in the state of Tennessee for a period of time of not less than three (3) years nor more than ten (10) years. Notwithstanding any other provision of law to the contrary, the fourth or subsequent conviction shall be a Class E felony punishable by a fine of not less than three thousand dollars ($3,000) nor more than fifteen thousand dollars ($15,000); by confinement for not less than one hundred fifty (150) consecutive days, to be served day for day, nor more than the maximum punishment authorized for the appropriate range of a Class E felony; and the court shall prohibit the person from driving a motor vehicle for a period of five (5) years. For the provisions of the preceding sentence to apply, at least one (1) of the violations of § 55-10-401 must occur on or after July 1, 1998. After service of at least the minimum sentence day for day, the judge has the discretion to require an individual convicted of a violation of the provisions of §§ 55-10-401 - 55-10-404 to remove litter from the state highway system, public playgrounds, public parks or other appropriate locations for any prescribed period or to work in a recycling center or other appropriate location for any prescribed period of time in lieu of or in addition to any of the penalties otherwise provided in this section; provided, that any person sentenced to remove litter from the state highway system, public playgrounds, public parks or other appropriate locations or to work in a recycling center shall be allowed to do so at a time other than such person's regular hours of employment.
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That link might not be exact, but it'll get you in the neighborhood.
Looks like it's not a felony until the 4th offense, assuming no other circumstances like fleeing the scene, homicide, etc.