98 Chevy S10 4 cylinder motor noise/issue?

My truck is making a dull clicking noise that gets better/worse depending on the RPM. It has significant loss of power, yet someone I was talking to said I should check the plugs and wires because to them it sounded like it isn't firing on all cylinders. I had already changed 3 of the 4 (front one is a pain because I have to move/remove the A/C thing. Could it just be not firing on one cylinder? Before talking to that guy, I was convinced it could be a lifter problem. Any advice?