4mbit is more bandwidth but probably on a smaller port like 10mbps. 500GB means a lower amount but probably on a 100mbps. Depends if you need to burst a lot or not.
usually, when going with unmetered, it's capped at that speed. It's ended up with slower performance. And 4 mbit cannot give you 1300GB transfer/month. In theory, yes, it can. But from my real experience, it is 200GB or less for one mbit
i would go with 500GB. because you don't know when the bandwidth is used. and also,in practise,4mb unmetered is too less,you cannot get over 350GB.
4 Mbit/s = Max. 1265.625 GB over 30 days. Note that 4 Mbit/s = 500 KB/s max. This will disadvantage you at peak times, as your visitors will have to share a 500 KB/s pipe. I would personally opt for 500GB bandwidth, but again, check what the burstable port speed is. Jay
I have not heard anyone offering 4Mbps unmetered, it would be very hard to do without special equipment and software.
I'd suggest going with the 500GB/M for the first few months. Setup MRTG on the server and monitor the bandwidth usage on the interface. This will let know know if you need the to be able to burst to what they allow you to burst to.