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Now a little magic which is not that obvious (and thus often overlooked): Hover your mouse over the tag, it will reveal a short description and some links. Click on the info link, and find your answer :)
In short here: When manufacturers speak of "ROM", they quite often use this term in a wrong way. The correct meaning of the term is Read-only memory, which is clearly not the case for those 4 GB. What they rather are is the all-over storage contained in the device: system (read-only) + internal + device storage. For details, please see the already mentioned tag wiki.
Manufacturers use the term ROM in a wrong way to show off with "big numbers", unfortunately – which causes a lot of confusion amongst customers.