I contacted TP-LINK support before, they said there is no big difference between V4 and V5.

Thank you, I will buy this router and I found only V4 in the market.

From wikidevi , I don't see any difference !
The only difference seems into the firmware...

Changing the versions for almost nothing is a totally stupid marketing strategy.
Further more, it's difficult to buy a TP-Link product because these versions number are rarely displayed by the reseller (amazon, etc.)...

I can tell you that V4 has a big and unresolved ( and also confirmed) problem with NAT boost. If from any reason the PPPoE connections drops, the entire NAT crash and the router barely function until a restart. This not happen with hardware NAT disabled. They confirmed and until now didnt find any solution for this problem. The problem is present only on V4...

@AntonyT 

I know this is old but for anyone looking for this info: Archer C7 has dropped 1 USB port (V4 had two, V5 has one). 

I guess it doesn't really matter - it's one unusable USB port vs previously 2 unusable ports.