There are no firm commitments to any future Spiderman films at this time, though there is hope. Sony is taking a wait-and-see approach. There's no doubt that this film will do tremendous box office, but Sony will be gauging fan burnout with this one. As has been reported, the budget for SM3 is expected to be in the area of $250 million. To do SM4, the stakes will need to be raised yet again, and that means an even bigger budget. $300-325 million is not out of the question.
Sony will be attempting to determine whether Spiderman as a franchise still has all it's legs underneath it. Make no mistake, they are concerned by the fact that SM2 dropped in total box office from SM1. They'll be looking not only at box office, but also at what future interest the core fanbase and general public will have now that Venom has been introduced. Can the series survive and thrive despite the likelihood of stepping back to "lesser" character villains for future installments?
The fact that each subsequent entry into this series is expected to be bigger and better(and more expensive) is a significant reason why Raimi buckled and brought the symbiote/Venom story into SM3. He, along with everyone else at Sony, just doesn't know for sure if there will be a SM4. So they're going for broke with this one. If SM3 can't bring in at least $375 million with the promise of the Venom storyline and the fx of Sandman, then it's unlikely that anything they could do in SM4 would hit those kinds of numbers. They need to know that for $300-325 million, they have a good chance of making a few (million) bucks on the deal.
We will not hear officially whether or not SM4 will happen until at least a month after the release of SM3, and it may not come until after the dvd is released.