MB means 1 million bytes of memory
GB means 1 billion bytes of memory
There are 1000 MB in each GB
Therefore, if you have 4GB of memory it's the same as 4000MB of memory.
To be more precise: 1MB = 1024 x 1024 bytes, and 1GB = 1024 x 1024 x 1024 bytes.
So 4GB of memory = 4 x 1024MB = 4096MB of memory.