Depends on how you count.
Strictly speaking a MB or megabyte is 1000 KB (= kilobytes). 1 KB is 1000 bytes. So 4 MB, is 4,000,000 bytes. This way of counting is often used by storage manufacturers.
Softwarewise, a MB usually means a mebibyte, or mega binary byte, sometimes abbreviated as MiB for clarity. 1MiB = 1024 KiB (=kibibytes or kilo binary bytes). 1 KiB = 1024 bytes. So in your case, 4 MB (or more accurately 4 MiB) = 4*1024*1024 bytes = 4,194,304 bytes.
As you know, current computers use binary language (i.e. base 2). 1000 isn't a power of 2, but 1024 is: it is 2 to the 10th power. Hence the preference of choosing 1024 instead of 1000 for software related amounts.
The difference between 4 MB and 4 MiB may seem insignificant, but as the amounts increase, the difference becomes more important. E.g., when a project leader requests 500 GB of diskspace (i.e. expecting 500 GiB or 536,870,912,000 bytes), and you buy 500 GB worth of storage (i.e. 500,000,000,000 bytes) , you'll be 36,870,912,000 bytes or a little over 34 GiB short.
If your not sure whether the person uses the decimal (1000) or the digital (1024) MB, simply ask. Better safe than sorry.