I was thinking to myself and came up with something on this.
So, from Origins to Awakening the companion that tied in was a male dwarf warrior (Oghren) then from Awakening to DA2 the companion was a male human mage (Anders) then from DA2 to Inquisition it was a male dwarf rogue (Varric) so with this taken into consideration, I'd say the companion to crossover into DA4 should be a female one since it's well overdue and given history I'd say that whoever it is could be any class since there have been one of each so equal chances but given the lack of elf companions that have tied into two games, I think we might want to place a wager on Sera being the companion that ties in during DA4 since she's an elf and a female companion so there's that.
BUT- if they keep the sort of pattern I see (Dwarf>human>dwarf) then the next one should be a human that's possibly male again since all of them have been male thus far and if this is a pattern (warrior>mage>rogue) it's bound to repeat from the beginning so maybe a male human warrior so chances with that would be Blackwall (although I personally doubt that because he was one of the least liked characters in DAI for some reason (yes I know he was a liar, but so was Solas and he even took Inkys arm not to mention all of the fade bull he got to pulling and you guys still really love him)) and the only other human male warrior that pops into my head would be Krem which could make a bit of sense given the game will apparently be set in Tevinter (maybe he pops up trying to set things right with his family? Idk about him though because he could possibly be dead depending on some of the Inquisitors choices)
Anyway, I would just like to hear your opinions, thoughts, etc.