Why don’t Christians keep the 4th commandment if they keep the others?
The Ten Commandments show us our failure, sinfulness and need of the Savior. It is impossible for anyone to keep them (see 1 Tim. 1:8f; Rom. 7:1f; 2 Cor. 3:6f; Gal. 3:10; 5:1f). Never in the NT epistles are believers told to keep the letter of the law, but rather through our new life in Christ and the ministry of the Spirit, we are to produce the righteousness of the law, i.e., the spirit of the law—its moral character in the sense of love for God and for others (see Matt. 22:34-40; Rom. 8:1-4). Thus, we are not under the letter of the law but the spirit of the law.
Historically, the early church assembled for fellowship and worship on the first day of the week, Sunday, not on the Sabbath. Likewise, in the epistles (after Pentecost) we are never told to tithe, but to give as we are prospered from a willing heart. The focus and emphasis of the NT epistles (those books written to guide the church, the body of Christ) is on learning to walk by the control or leading of the Spirit rather than by a set of legal commands.