i7 4610M is the best it can fit, at least according to dell but you should be able to go for the i7-4712MQ which is the quad core counterpart and thus faster.
The problem with upgrading laptop cpu's is the cooler. If the cooler was not designed for a higher wattage cpu you can hit overheating problems. It is common for different models of laptop in the same range to have different coolers based on the CPU in that model. They will use the cheapest cooler they can get away with