In the Old Testament we learn how that He selected a man, Abraham, and made promises to him regarding Israel and the world (Genesis 12:1-3; Galatians 3). This plan included a nation, Israel, which would evolve from Abraham and the family of Jacob. The descendants of Abraham would become a great nation, and also, from Abrahams descendants Christ would come to bless the world.
In the Old Testament we have God revealing His plan and how it was to be accomplished. God gave them a law (Exodus 20). The nation of Israel is formed and given the land of Canaan (Exodus Joshua). He reminded them that this was on the condition that they kept His law (Deuteronomy 28-29). If they were obedient to God then they would keep the land; if they disobeyed they would lose the land. They retained the land for several years, but because of their disobedience they were taken into captivity. God had prophesied that they would be in captivity for seventy years and then some would return to the land of Canaan (II Chronicles 36:21-23; Jeremiah 29:10; Nehemiah 1:8-9).
God reveals to us in the Old Testament how he preserved Israel in preparation for the coming of Christ (Messiah). All of this is revealed in the Old Testament and that part of the revelation was complete. There was no more to tell regarding that part of Gods plan.
Why then after 400 years did He speak again? In the Old Testament only part of the plan was revealed, but full revelation was not complete. The promises of God regarding the physical promises to Israel were all fulfilled, but the spiritual promises were yet to be fulfilled. There was the promise of a new law (Jeremiah 31:31-34), the promise of a new kingdom (Daniel 2:44), the promise of new blessings (Genesis 12:1-3; Galatians 3), the promise of the Christ being the seed of Abraham that would bless all nations (Galatians 3:8, 16; Acts 3:22-26). All of these truths were to be revealed later.
After 400 years God spoke in the fullness of time (Galatians 4:4) to reveal the fullness of His plan (Hebrews 1:1-2). He tells how forgiveness and salvation comes through Jesus Christ. Abraham, the family of Jacob and the nation of Israel were all part of Gods plan. It was through these that Christ came (Romans 9:5). The New Testament is the final or complete revelation from God (James 1:25; II Peter 1:3; II Timothy 3:16-17).
Could there be another revelation after the New Testament? The Bible speaks of only two, the Old and the New (Hebrews 1:1). There will be no new revelations. The Old Testament spoke of the New coming, but there is NO reference in the New of another. The New Testament completed Gods revelation, so there is nothing more to be revealed (II Timothy 3:16-17). At the close of the Old Testament the provisions for mans salvation were incomplete. The New Testament finished Gods revelation to man.