Router Latency, when network have different bandwidth

I have a question: I have a 4Mbit Connection on the one side (call it Side A) and a 1Gbit connection on the other side (side B)开发者_如何学C of a router. If now a packet of 1500 bytes is coming in on Side A. Does the router wait until it received everything of the packet (all the 1500 bytes) on Side A until it forwards it to Side B, or is it sending it in chunks?

Sorry if the question is too fundamental, but I really did not find a good answer to this question.

if it's an IP router (so called 'layer 3 routing') it gets the whole packet (which can be less or more than 1536 bytes), verifies the checksum, and then forwards to the next hop.

'layer 3 switches' (in theory it would be an oxymoron, in practice it's not all uncommon) claim to do 'routing at the speed of switching', which might hint at starting to retransmit before receiving the whole packet; but AFAIK, even layer 2 switches (also known as 'switches') do get the whole ethernet frame before retransmitting it (at least to be able to discard bad frames). So I guess it means that it does all the table lookups as soon as the header is received, so it's ready to retransmit as soon as it's finished. when doing layer 3, the switch tables include IP address in addition to MAC addresses.