Tai-San introduced the pentacle as a symbol that would identify all members of the tribe back in Ep.6 of S2. It was universally on the left hand. Above we see Tai-San's hand and Bray's. We guess it's meant to be permanent, like a tattoo. As time went by it seems Cloud9 forgot to apply it to all the cast, no doubt assuming we'd all forgotten about it.
Now we're into Season 4, it seems the Mallrat pentacle will be important again, and the Mallrats have all got them back. The only problem is, in many cases, they've reappeared on the wrong hand.... Then Mouse has one, albeit on her left hand, despite the fact she's only just arrived and hadn't formally been received as a member of the tribe. She'd need to ask, or to agree, but to date she hadn't uttered a word - except to her brother. Then to cap it all, Pride gets one on his right hand in Ep. 14..
I got bored after Episode 16, but that was enough to prove the point.
  end of S3         S4   end of S3         S4
Amber none   Right Ellie none   Left
Lex none   Right Salene --   Left
Cloe none   Left - none - left Mouse --   None - left
Jack none   Right - none - right Pride none   None - right
Trudy none   Right Dee --   None - left
Amber: (S3: none) S4: right  
End of S3: When Amber left the City, there was no pentacle on her right hand. While Bray and Amber were traveling cross-country she went into labor. Fortunately there was a barn nearby and they made for that. Bray helps her to lie down on a bed of straw. There is no pentacle on her right hand.
The scene resumes in S4, and apart from her hairstyle instantly changing, she now has a right-hand pentacle.
Lex: (S3: none) S4: right - none - right - none - right  
S3 :Ep.52. Lex escorts Bray and Amber out of the City at the end of S3, having been banished by Ebony. Lex sends them on their way with a small amount of food. Lex is left standing with a small bag of money also meant for Bray and Amber. Of course it's worthless outside the City. pentacle: none S4: Ep.1 The remaining Mallrats are stunned at how many people are now missing. Lex has lost Tai-San. pentacle: right
S4: Ep.15 Lex's second meeting with Siva in the Luv shack. If there's a pentacle there, it's so faint it doesn't count. pentacle: none S4: Ep.15 Lex has toothache, but says it's better than Pride's disgusting herbal cure. Oh, and the Mallrat pentacle is back: pentacle: right
S4: Ep.16 After Dee has pulled out the bad tooth, he makes his groggy way to his third meeting with Siva. She thinks he's faking sick as a pretext for dumping her. pentacle: none S4: Ep.16 Lex staggers back to the Mall and passes out at the top of the stairs. He's seriously ill. The pentacle? Back. Right hand.
Cloe: (S3: none) S4: Left - none - left - none - left  
Above left is a picture from the end of S3 showing Cloe with no pentacle at all. Right pic: she now has a LH pentacle. This is taken 9 min. into S4:Ep.1.
S4: Ep.4 Cloe appears in a short scene (16 seconds) where she's about to change Lex's bandages. On the way she passes Pride, and it's so obvious she has the hots for him. But note, no pentacle anywhere.
Oh, and that messy doodle on her chin? Where did that come from? Where did it go?... (see also)
S4: Ep.4 With that blue-smudge chin in the past, Cloe has grown back her left-hand pentacle.
S4: Ep.14 Ved visits Cloe to return her Teddy - and to ask her on a date. But no pentacle. S4: Ep.15 And she's got it back. t Left-hand pentacle.
Jack: (S3: none ) S4: right - none - right  
S3:Ep.50 Jack has just escorted Cloe to the Mall when he's spotted by Amber. His hands: no pentacle S4:Ep.1 Jack reluctantly goes with Ellie who's keen to get a story for the Newssheet. pentacle-right
S4:Ep.3 Jack creates a diversion to save Ellie from marauding Technos. She's on her way to the farm hoping to find Alice. pentacle-none S4:Ep.3 They've arrived at the farm. No Alice, but Jack's pentacle has grown back. pentacle-right
Trudy: (S3: none) S4: right  
S3:Ep.50 Trudy is shocked to hear that Tai-San has (falsely) promised to take Brady to the Guardian. Her right hand: no pentacle. (And none on her left either) S4:Ep.1 Trudy leaves the City to find Amber. pentacle-right.
Salene: (S3: n/a) S4: Left - none - Left - none  
S4: Ep.2 At the end of S3 Salene rode Apollo out of the City to go search for Ryan. She returns in S4:Ep.2 without Apollo (Why?). Not far from the Mall she finds a homeless and sick girl, Mouse, and decides to help her. Salene has a left-hand pentacle. S4: Ep.2 Salene leads Mouse back to the Mall. But now her left hand has no pentacle. Not long after, Salene cares for Lex after Ved zapped him. And still no pentacle.
S4: Ep.3 Salene brings order to a very disorderly tribe leaders' meeting. Left-hand pentacle. S4: Ep.6 Ebony leads an all-out attack on the Techno base. It's what the tribe leaders demanded, though she was against it. Salene: no pentacle
S4: Ep.6 The Mallrats were no match for the Technos. Only Ebony knows it was a set-up. Salene Bandages her injuries. Salene: left pentacle back again.  
Ellie: (S3: none ) S4: Left  
S3:Ep51. Ellie is optimistic about the future. Luke definitely isn't and (unknown to Ellie) plans to leave the City. No Pentacle S4:Ep.1: Ellie covers her ears when the Techno pyrotechnics begin. Pentacle: Left
Mouse: (S3: --) S4: None - left - none  
S4: Ep.2 This is the first time we see Mouse. Salene finds her while returning to the City after a fruitless search for Ryan. Mouse has no Mallrat pentacle as yet. Of course not! She's not a Mallrat. But you never can tell with the Cloud9 makeup department! None S4: Ep.4 Mouse and her brother Charlie will only eat when they think no one is watching. Mouse's right hand is still pentacle free. Pentacle: none
S4: Ep.6 The Mallrats have joined a Citywide attack on the Technos leaving Cloe to baby-sit Mouse and Charlie, who are rummaging Pride's stuff. Mouse: no pentacle S4: Ep.6 Having picked through Pride's bag, Mouse and Charlie find Cloe's love letter to Pride and make a paper plane. Mouse now has a left-hand pentacle.
S4: Ep.10 Mouse's pentacle has gone. - none  
Pride: (S3: none) S4: None - right  
S4: Ep.14 OMG! Pride's got one now - you've gotta be kidding! He's not even a Mallrat.
Dee: (S3: none) S4: None - left  
Dee, erstwhile Mozzie, also, apparently, becomes a Mallrat. Moz herself would be turning in her... wherever she is. This is taken from Ep.28.