If the PS4 does use the same Cell processor as the PS3 than there could be a huge possibility that the PS4 will be backwards-compatible. Maybe once this HD frenzy is over and people stop giving a rat's behind, Sony will do what the Wii did and go for a more cost-effective, gameplay rich console rather than the behemoth media centre they have created!
Since the Cell processor technology would have been researched into fully and the cost of manufacturing it becomes significantly low, the PS4 would not be as expensive as the PS3 during launch.
Also, if Sony decides to keep the Blu-Ray drive on the PS4 and if the Blu-Ray technology gets more recognized by the average Joe than that would also cut down costs for the PS4.
My thoughts are that Sony will decide to release the PS4 once the PS3s market rises and becomes saturated... This will insure that they will get a wide fan-base like the PS2 due to cheap price and backwards-compatibility.
I don't know if any of that made sense... My brain is cooked!