After 1. d4 Nf6 2. Bf4 e6 3. e3 d5 4. Nd2 c5, Black opts for a setup reminiscent of the Queen's Gambit Declined (QGD). This approach aims for a solid yet flexible structure, central and queenside counterplay, and harmonious development of pieces:

1. Solid Central Pawn Structure: With ...e6 and ...d5, Black sets up a strong pawn chain in the center, often making it difficult for White to easily break through.
2. Challenging White's Center: The move ...c5 directly targets White's central d4 pawn. This pawn break aims to destabilize White's pawn center and create counterplay on the queenside.
3. Development and King Safety: Black can continue with natural developing moves such as ...Nc6 and ...Bd6, exchanging off White's active dark-squared bishop if possible. Following up with ...O-O ensures the king's safety.
4. Activating the Light-Squared Bishop: One of Black's challenges in the QGD setup is the passive light-squared bishop on c8. Black can consider plans such as ...b6 and ...Ba6, or even a timely ...e5 pawn break, to free this bishop.
5. Queenside Counterplay: Using the semi-open c-file, Black can look to increase pressure on the queenside. Rooks can be quickly deployed to the c-file, and pieces like the knight on b8 can be developed to c6, supporting the ...c5 break and increasing pressure on d4.
6. Central Breaks: Depending on White's setup, Black can consider the ...e5 break in the center, especially if the opportunity arises where the d4 pawn can be targeted.

. 1. d4 d5 2. Bf4 Nf6 3. e3 e6 4. Nd2 c5

Wikipedia has related information at London System.

• chess.com - Indian Game: London System - Chess Openings
• 365chess.com - London System Opening Complete Guide - 365Chess
• stackexchange.com - opening - Battling the London System - Chess Stack
• Chess Opening Crash Course: The London System
• Win with the London System. 2005. Sverre Johnsen, Vlatko Kovačević. ISBN 1-9046-0035-2.