4...Bb4+
Catalan Opening · E01 · you play White. 1.d4 Nf6 2.c4 e6 3.g3 Bb4+ 4.Bd2 Be7 5.Bg2 d5
About the 4...Bb4+
The Bogo-Indian check 3...Bb4+ against the Catalan is usually met by Bd2. After ...Be7 White has gained the tempo back and plays a normal Catalan with the bishop on d2, which is well placed for Bf4 or Bb4 ideas later.
Playing the Catalan Opening as White? The 4...Bb4+ (E01) begins 1.d4 Nf6 2.c4 e6 3.g3 Bb4+ 4.Bd2 Be7, and is one of 3 lines you can practise here. You study the moves first and then play them against the computer; every wrong move gets a short explanation. The page continues past the opening with middlegame plans, typical endgames and master games played with the 4...Bb4+. Use it to build your set of lines to play with White, and test what you remember with speed training or the random-position drill.