4 pairs are in Cat5 cable so that any or all pairs can be used for something.
* 10BASE-T Ethernet uses only 2 pairs (1/2 and 3/6).
* 100BASE-TX Fast Ethernet uses only 2 pairs (same ones as above).
* 10/100 ports use only 2 pairs (same ones as above).
* IEEE 802.3af Power over Ethernet can use the unused 2 pairs (4/5 and 7/8) to deliver power over the data cable to 802.3af-compatible devices like Ethernet or IP Phones, Wireless Access Points, etc.
* 1000BASE-T Gigabit Ethernet uses all 4 pairs (1/2, 3/6, 4/5, 7/8).
Strictly speaking, 2-pair Cat5 cable is NOT Cat5 cable. True Cat5 cable always has 4 pairs. You can have cable with 2 pairs and similar transmission characteristics to Cat5, that works with 10, 100, or 10/100 devices only. But you can't call it a Cat5 cable run because it won't deliver PoE or 1000BASE-T. If you put a cable scanner on there and run a Cat5 certification test, it will fail the test because it's missing 2 pairs.
If it is 2-pair-only cable, and has an equal or greater number of twists per foot (English) or meter (metric) than true Cat5 cable, then it probably won't perform any different than true Cat5 cable for 10, 100, and 10/100 connections.
If it's 4-pair Cat5 cable, and you run 2-pair for a normal 10, 100, or 10/100 connection and also punch down the unused pairs in the same cable for another 10, 100, or 10/100 data outlet, that may work but it is bad cabling practice. Also, you can't claim that the cable run is Cat5, because the standards require all 4 pairs to be terminated on the same data port. Effects on performance: you may run into crosstalk problems, where data is corrupted as it travels on the wire. The link's performance can be degraded because the devices on each end may have to send frames/packets multiple times to get them through once.
Hope this helps.
