I know now why my data usages is over the limit. The wifi keeps dropping off. I just bought a new Samsung S4 and thought that my old S3 was the problem. Is it a router problem or a S4 problem? There seams to be lots of discussions on the S4 having connection holding problems.
Go into your wifi settings and make sure smart network switch or download booster are not on, these use a combination of cellular data and wifi to boost download speeds. Also in the advanced settings within the wifi settings make sure you have the "Keep wifi on during sleep" set to Always.