Distracted by a Tower request to adjust the transponder; Captain of a Cessna 550 crosses the active runway without a clearance.

Narrative

We were taxiing out for takeoff and were cleared to taxi from the FBO to Runway 4 at HOU. As we approached Runway 4; Ground Control called us to ask us to turn our squawk on. I looked over and we were already squawking the proper numbers on both transponders. Rather than let the First Officer handle his job (which he was very capable of doing); I became distracted and taxied across the approach end of Runway 4 to get in line behind a commuter jet holding short of another runway; I believe Runway 35. Of course; my mind was telling me he was holding short of Runway 4 and we were going to follow him. Fortunately; there was no other traffic on the approach to Runway 4 and; therefore; no conflicts with anyone else. This is 'proof positive' that no matter how many hours/years of experience one has; if you lose your concentration for only a moment; it could certainly turn out much worse than this -- a valuable lesson learned by a very experienced pilot. Also; I might add; a very lucky one! This was discovered by Hobby Tower; they told us that we had crossed Runway 4 and to do a 180 degree turn and cleared us for takeoff from the other side. Since there was no conflict -- no harm/no foul; but it is my duty to report this because if this report were to help prevent just one conflict/accident; then it is certainly worth my time.