Pins 4, 5, 7 and 8 are not used for Ethernet (10 Mbps) and FastEthernet (100 Mbps), but are used for GigabitEthernet and 10G Ethernet. This is the same for both straight through and crossover cables.
Remember that in modern switches, for all types of Ethernet, because of auto MDIX, either a straight through or crossover cable can be used. The devices will electronically decide which pins will be used for sending and receiving.
Power over Ethernet (PoE) can use the unused pairs of wires (for 10 and 100Mbps Ethernet) for sending DC current to power for PoE devices, but the technology is not limited to just using those wires. As you can see from the PoE lesson below, both power and data can be sent over the same pairs of wires without any problem.
So to answer your specific question, those unused pairs are simply not used for 10 and 100 Mbps Ethernet.
I hope this has been helpful!