When your web hosting company says you are allowed 4,000MB of data transfer per month, they mean that you cannot transmit more than 4,000,000,000 bytes of data to your web site visitors without paying an additional fee. Some web hosts use the term "bandwidth" to mean the same thing, although this is not strictly accurate. One megabyte (1MB) equals one million bytes. One gigabyte (1GB) equals one billion bytes; you often see the above data transfer limit written as 4GB.
Understanding Data Transfer
How can you tell in advance what your web site's data transfer needs are going to be? Let's say that your web site is made up of four web pages, each of which is 5,000 bytes in size, and twenty images, each of which is 50,000 bytes in size. Let's also assume that when a a typical visitor comes to your site, they look at half of your pages and half of your images.
That works out to 10,000 plus 500,000 equals 510,000 bytes per visit.

Divide your 4,000 mb -- that's 4,000,000,000 bytes, or 4 gigabytes (gb) -- by 510,000 bytes and you get 7,843 visits. That's how many visits to your web site you can receive each month before you run into the limit imposed by your hosting company. After that, you typically pay a surcharge.

Of course, if your web site does not contain many large images, your requirements will be much lower. For instance, if your site is made up of four 10,000 byte web pages but contains only five images that average just 2,000 bytes each, and your typical visitor looks at half of this material, that works out to 160,000 visits.

Of course, these are approximations. To be sure what your site is doing, you should look at your web log analysis reports. Good programs of this type can actually tell you how many bytes you have transferred this month. You may already have such reports, courtesy of your web hosting company.