World at War is not the 5th CoD and yet some people call it "CoD 5."
Anyway, Call of Duty 4 was the last title to feature a major number in its name. Every game after was either a sequel or a spin-off with separate titles.
Since Call of Duty releases yearly, do you think they'll run out of ideas and make a Call of Duty 5? Maybe that one will go back to WW2.
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No. I doubt much of CoDs current playerbase has even played CoDs 1-3 or even CoD 4. So I think it would confuse them if a CoD 5 comes out 7+ years after CoD 4.