No. Not all the i5s are Built with 4 physical cores. Even i7s have dual core versions.
i3,i5,i7 all the processors have dual core versions. so be careful to choose a processor which have 4 physical cores. Most of the laptop processors, even i7 are equipped with dual core processors but they are hyper-threaded and shows as 4 physical cores in the device manager. when you go to the device manager, if it shows 8 processors, then it can be sure indication of 4 physical cores. But should keep in mind, some i5 only have 4 physical cores and not hyper-threaded.
Best way is to search the processor name on the internet and to check with the intel website.
