Contradiction with Numbers 1:49
Numbers 4:23 instructs a census of Levites for service, whereas Numbers 1:49 commands not to count the tribe of Levi among the other Israelites.
Numbers 1:49: Only thou shalt not number the tribe of Levi, neither take the sum of them among the children of Israel:
Contradiction with Numbers 8:24-25
Numbers 4:23 designates Levites to serve from age 30 to 50, but Numbers 8:24-25 states that Levites begin service at the age of 25.
Numbers 8:24-25: This [is it] that [belongeth] unto the Levites: from twenty and five years old and upward they shall go in to wait upon the service of the tabernacle of the congregation: [to...: Heb. to war the warfare of, etc]
Contradiction with 1 Chronicles 23:24
Numbers 4:23 specifies the age of service as 30-50, yet 1 Chronicles 23:24 states David set the age at 20 years.
1 Chronicles 23:24: These [were] the sons of Levi after the house of their fathers; [even] the chief of the fathers, as they were counted by number of names by their polls, that did the work for the service of the house of the LORD, from the age of twenty years and upward.