4.Ba4 (C70: Main Line Ruy Lopez)
- White maintain attack and possibility pinning Black Knight against Black King, if Black move d pawn.
- Retreating White Bishop into Bc4, Bd3 and Be2 is inferior moves, because White Bishop can go into that squares earlier without going into Ruy Lopez.
- Retreating White Bishop into Bc4, Bd3 and Be1 give Black free a6 move.
Black Responses
- By seeing White last move (4.Ba4), Black assume that White didn't want to exchange Bishop with Knight immediately.
- So, Black delayed b5 move and try to attack undefended White Pawn on e4 with his/her Knight.
- By playing 4...Nf6, Black also developing g1 Knight into best starting development square.
- By playing 4...Nf6, Black moving piece which required to be moved, in order to castle on the kingside.
4...d6 (C71: Modern Steinitz Defense)
- This move is improvement to Old Steinitz Defense.
- By playing Modern Steinitz Defense, Black plan is similar to Old Steinitz Defense (protect e pawn and open way for c8 bishop move out from back rank).
- The advantage playing Modern Steinitz Defense then playing Old Steinitz Defense is Black ready any time to break pin using b pawn.
- The chance of breaking pin any time, give Black more freedom in choosing response against White.
4...Bc4 (C70: Ruy Lopez, Classical Defense Deferred)
- Black try to attack White Pawn on f2.
- By playing 4...Bc4, Black developing Bishop to c5 square while at the same time facilitate castle on the kingside.
- This move named Classical Defense Deferred, because this position similar to Ruy Lopez Classical Defense.
- The difference, Black delayed (deferred) playing Bc4 in third move.
- Black play a6 on third move and Bc4 on fourth move.
4...Nd4 (C70: Ruy Lopez, Bird Defense Deferred)
- This move name Ruy Lopez Bird Defense Deferred, because this position similar to Ruy Lopez Bird Defense.
- The difference, Black delayed (deferred) playing Nd4 in third move.
- Because White Bishop no longer in b5 square, so Black Knight only attack White Knight on f3 square and c pawn.
- The drawback of this move is potentially make double pawn on d file, if White play 5.Nxd4 and Black response with 5...exd4.
4...b5 (C70: Ruy Lopez, Caro Variation)
- Black hunting White Bishop with b pawn.
- It better to delay b5 move, because by using Morphy Defense (3...a6) Black now can assume that White didn't want to play Ruy Lopez Exchange Variation.
- It better moving piece which help king to castle on the kingside (moving f1 Bishop or g1 Knight) or protect e pawn with d6.
