A 10% tithe on $4,000 per month is $400 per month and $4,800 per year.
Gross vs net
On gross income of $4,000 the tithe is $400. Christians who tithe on net (take-home) pay give ten percent of what lands in the account instead — Proverbs 3:9 ("honour the LORD with your firstfruits") is the usual argument for tithing on gross.
Related tools
Tithe Calculator · Tithing Hub · Malachi 3:10 meaning · Generosity Calculator