Arlanda recently opened a new airside interconnect between T4 and T5 which makes the airport rather easier to use. It means that T4 and T5 effecticely become a single terminal... it's where most international flights (except Oneworld) depart from
Usually ARN is highly efficient... although some areas can at times be very busy with people - if you have a couple of spare hours, the newer part of T5 is much more comfortable (if you see dark wood flooring, you are in the right place). I suspect the current security screening chaos will stop by September when Swedes return from the beach.
Trains to the city centre are very expensive albeit fast. The coach by comparison is surprisingly good