You have more problems than just selecting 4 wires. Each pair of wires from the 100 megabit Ethernet is a balanced pair, and coax is unbalanced. Also, the coax impedance is probably 75 ohms (if I remember correctly) and the twisted pair is around 120 ohms. So you would need a transformer for each pair to match the impedances and handle the balanced to unbalanced conversion. Finally, these transformers would need to be small and broadband to avoid unintentional impedance mismatches.
Like the others have suggested, just use the coax as a pull wire for Cat 5e.