first semi finalist of IPL 4
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MI
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CSK
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RCB
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KKR
In IPL 2011 (Season 4), RCB (Royal Challengers Bangalore) topped the league table with 21 points, qualifying first for the playoffs. They were the first team to secure semi-final qualification by finishing at the top of the standings. MI, CSK, and KKR all qualified later.
In IPL Season 4 (2011), Royal Challengers Bangalore (RCB) finished at the top of the league table, which meant they were the first team to secure a place in the playoffs and went directly into Qualifier 1 (effectively the first-confirmed semifinal-stage berth), later advancing to the final where they lost to Chennai Super Kings. MI, CSK, and KKR also made the playoffs that season (via Qualifier 1, Eliminator, and Qualifier 2 stages), but RCB's league-topping finish made them the first team to lock in a semifinal/playoff spot, matching the question's phrasing about being 'first'.