Elementary may have proved to be mildly diverting procedural fun, rather than a dramatic match for its British counterpart Sherlock, but among the US show's positives has been the return of Lucy Liu to TV screens as the icy, occasionally amused Joan Watson.

After a period where she became best known for voicing a snake in Kung Fu panda, it's good to see Liu back centre stage. But Watson isn't her only current TV role that demands attention – she's even better in the unfairly overlooked cop show Southland, which begins its fourth season on More4 this week.

As Officer Jessica Tang, Liu takes a character who could have been seen as little more than a bundle of quirks with a troubled past and makes her entirely understandable. Hampered by assumptions about her slight build and seeming fragility, Tang constantly has to prove herself to both the public and her fellow cops. It's the sort of performance that could appear one-note were it not for a strong script and the surprising warmth Liu brings to her character, ensuring that we never fail to glimpse the shell-shocked woman behind the officer.

Where Watson's relationship with Jonny Lee Miller's Sherlock often seems to be for dramatic purposes, Tang's relationship with her patrol partner John Cooper – whose fitness for street duty she was initially supposed to be assessing – is complex and real, from their initial distrust through their tentative attempts to build a partnership.

Southland itself also has more depth than the average cop show. When it first started it was dismissed as a by-the-numbers procedural; little more than a chance for the OC's Benjamin McKenzie to demonstrate that there was more to him than a well-defined set of pecs. Yet as the first season progressed it became obvious that there was a great deal more to Southland.

For a start there is the show's tone. Where many cop shows, on both sides of the Atlantic, feel almost too perfect – their storylines neatly rounded off, their edges sanded down – Southland has cultivated a rougher feel. You feel as if you've been dropped straight into the middle of a real-life tale; a perception that is further enhanced by the casting of off-duty LAPD officers and former gang members as extras.

Then there's the characterisation. From Mackenzie's all-too-eager rookie to Regina King's self-contained Detective Lydia Adams, Southland's cops are frequently tired, often overworked and always believable. They make mistakes, abuse people or are themselves abused; they find themselves in impossible situations, unsure whether their decisions will prove right or wrong.

The show is also unflinching in its examination of race relations in Los Angeles. In the first season, Adams makes the point that a murder case would have been front-page news had the victim come from wealthy, predominantly white Brentwood rather than South Central; later episodes deal with the segregated nature of LA and examine officers' racial attitudes towards each other.

For all its clever camera work and snappy direction, Southland's strength comes from the fact that it is a well-scripted but curiously old-fashioned drama at heart. Creator Ann Biderman wrote for NYPD Blue while John 'ER' Wells is among the producers – unsurprisingly the show values characterisation and offsets its more serious moments with an offbeat humour. Yet the drama it most resembles is less these past hits than the granddaddy of this kind of ensemble cop show, Hill Street Blues.

Like Hill Street, issues in Southland arise organically from the plot and characterisation. Like Hill Street it asks us to follow a group of disparate cops doing a hard, often soul-destroying job. And, most importantly, like Hill Street it convinces us to care.

• Southland returns to More 4 on Thursday 15 November at 10pm.