While future device support is speculative by definition, in the past porting GrapheneOS to a new device took a couple of months at least. And since Pixel 7 is coming out around October, that means I would expect GrapheneOS to come to it around December. (Just an estimate, could be sooner or later)
Pixel 4 is expected to lose support around October. So that means there would be an overlap of a couple of months where you wouldn't have full security coverage.
GrapheneOS has been offering extended releases a year after the end of life support, but note that these do not cover all the security patches (notably the firmware).
So the decision is up to you. Are you okay using a device without full security for a few months? If so perhaps wait for the 7. If that is unacceptable then you should get the 6 or 6a.