Welcome to the site Ghost, great to have you on board.
Yes, physically, the 400 & 650 are identical, however the 400 is sleeved in the cylinders bringing down the capacity from 650cc's to 400cc's. If you are looking for accessories, search for the 650R. They will all fit.
I'm assuming when you say 4km, you actually mean 4000kms on the clock? If it hasn't been serviced, then yes it is due, and probably is the cause of the oil light.
You do not have to use a genuine oil filter, K&N and many other brands also sell equivalent filters. I use the K&N KN-303. This is a one time use item, and should be disposed of each time you change the oil. You cannot wash out an oil filter and smack anyone who suggests it around the back of the head!!
With the bike upright, the oil should be around the half way mark on the sightglass. It doesn't have to be perfectly in the middle, but don't overfill it, that can be just as bad as not having enough oil in the bike. I'm not sure if it's available to you, but I use a semi synthetic oil from Penrite called HPR GAS 10 10W-50.
I hope that covers all your questions, but if there is anything else, please ask it here. I'm not on this site very often these days, but I'll check in soon.