Yes, but to what extent we don't quite know yet. Obviously when the PS4 launches, there wouldn't be that many games to offer as IGC.
From what I've read about PS World, there will be an $89/year fee to get PS+, Gaikai, and a few extra features. I'm guessing that they'll keep the $50/year PS+-only subscription as well and allow people to pay the $40 difference if they're currently subscribed to PS+. On the EU blog, someone asked about the future of PS+ if they extended their sub beyond the PS4 release date, and although the rep wouldn't say details, he did say "not to worry about PS+, it's not going away."
I think the best thing for Sony to do is leave the $50/yr PS+ as PS3+Vita and then if people pay upgrade to the 89/year subscription, that's when the PS4 titles would come into play (and it would be like the addition of Vita where PS3 content was unaffected). But we just don't know yet.
Like the Vita, it likely won't be available for the PS4 upon launch since there will be extremely limited options for the IGC, but eventually, it will happen. Of course, it is also highly likely that some form of the Gaikai streaming service could be attached to a PS+ subscription.
They were able to add Plus for the Vita. Also, on the introduction for the PS4, it was mentioned that they will try to make the Vita compatible for the PS4. So, if Vita has service, it is quite possible.
Yet, some people say that there wont be any cross-compatibilty. Also, it was mentioned that PS3 info might not cross to the PS4. This may grow some concerns. Yet, it is too early to finalize any proposals.