I've always wondered if the bandwidth rating of a switch is per port, or total capacity? For example, if I have PC1 and PC2 that are capable of saturating a 100 Mb connection plugged into a 10/100 switch, if I add PC3 and PC4 to the same switch and start transferring files from PC1 to PC2 and from PC3 to PC4 - will both sets of PC's have 100 Mb to use?
The 100Mbps speed of a switch port refers to the max one-way line speed of the port.
If you have two PCs on the switch and they are connected at 100Mbps full-duplex, you actually have 200Mbps of bandwidth between them - 100Mbps from A to B, and 100Mbps from B to A. (Some switch/NIC vendors take advantage of this on the packaging - don't be fooled!)
Switches are also limited on the backplane - they can typically handle up to a certain amount of bandwidth in terms of Mbps as well as a maximum number of PPS (packets per second).
Most of the time, you don't need to worry about saturating the backplane - you won't.
In your example, the traffic you're generating from PC1 to PC2 will go at max 100Mbps and the traffic from PC3 to PC4 will go at max 100Mbps - they won't interfere with each other.