“American Chopper” (8 p.m., TLC) enters its fourth season on a new network, and the Orange County Choppers prepare to open a vast new facility that will incorporate their garage, service center, retail store, offices and a display center where visitors can watch the guys work. Can a Teutul theme park be far behind?

The season opener features a contest between Paul Teutul Sr. and Paul Jr. to build a prototype bike that represents the future of the Chopper line. Dad opts to go old school, while his namesake crafts a more audacious and technically challenging bike.

It’s no secret why the “Chopper” show and franchise continue to grow. The bonds of affection between gruff patriarch Paul Sr., son Paul Jr. and court jester Mikey are as obvious as their shenanigans are amusing. And once you get beyond the many scenes of garage pranks and “let’s blow something up” field trips, this show boils down to craftsmanship, design and creativity.

¢ The staff of Sacred Heart Hospital breaks into song and dance on a musical episode of “Scrubs” (8 p.m., NBC), featuring a soundtrack of clever, silly, catchy tunes written by Robert Lopez and Jeff Marx (“Avenue Q”).

There’s nothing like a musical to make you look at a show in a whole new way. Some years back, when “Buffy the Vampire Slayer” was sliding into oblivion, creator Josh Whedon concocted a musical episode that was truly memorable. A lot of series have mimicked “The Singing Detective” over the years. Heck, a few years ago I heard that “The Powerpuff Girls” were going to come out with their own rock opera. I’m kind of sorry that never happened, or at least never aired.

Tonight’s “Scrubs” presents a fast-paced romp through the last half-century of Broadway musical styles, from big song-and-dance numbers to intimate solos under floodlight. Several numbers take their cue from “West Side Story,” and at least one show-stopper seems lifted straight from “Grease.”

The production numbers begin when a patient is admitted after insisting that everyone in her field of vision is bursting into song. But even though the musical is taking place only in her head, this “Scrubs” special manages to advance several plot points. Elliot wants her own place and Carla wants to return to work, and they don’t know how to tell the guys, because they’re too busy singing.

Tonight’s other highlights

¢ A victim on Earl’s list turns up dead on “My Name is Earl” (7 p.m., NBC).

¢ Daniel sulks, Wilhelmina schemes on “Ugly Betty” (7 p.m., ABC).

¢ The awkwardly outed Oscar returns with mixed emotions on “The Office” (7:30 p.m., NBC).

¢ On two episodes of “CSI” (CBS), Nick loses his car and key evidence (7 p.m.), a new member (Liev Schreiber) joins the team (8 p.m.).

¢ Derek can’t sleep on “Grey’s Anatomy” (8 p.m., ABC).

¢ Liz falls for a handsome guy from the cable news division on “30 Rock” (8:30 p.m., NBC).

¢ Abby’s overbearing mother (Sally Field, “Brothers & Sisters”) arrives unexpectedly on “ER” (9 p.m., NBC).

¢ Marin pitches in on “Men in Trees” (9 p.m., ABC).

¢ An accused killer acts as his own lawyer on “Shark” (9 p.m., CBS).

¢ “The Final 24” (9 p.m., Biography) recalls the grim end for Sid Vicious on Feb. 2, 1979.