A C550 CREW; TAXIING FOR TKOF AT KHOU; TAXIED PAST A HOLD SHORT LINE.

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WE WERE TAXING OUT FROM PARKING. GND CTL CLRED US TO TAXI TO RWY 4. WE HAD JUST DONE THE TKOF CHECK AND I (PNF) WAS CHECKING THE INSTS. THE LEFT SEAT PF WAS LOOKING OUTSIDE AND TAXIING. WE HAD JUST TURNED RIGHT ON 'G' AND THE PF POINTED OUT A RED 17/35 RWY SIGN. I ALSO SAW THE SIGN. HE SAID WE ARE CLRED TO CROSS RWY 17/35 AREN'T WE? I SAID YES WE ARE. I ALSO SAW THE 17/35 RWY SIGN. HOWEVER NEITHER OF US SAW A RWY SIGN FOR RWY 4/22. ABOUT THIS TIME GND CTL CALLED AND TOLD US TO CONTINUE TAXIING ACROSS RWY 4/22 SINCE THERE WAS AN ACFT ON A MILE FINAL; AND TAXI TO AND HOLD SHORT OF RWY 35. AT THE TIME HE TOLD US TO CROSS RWY 4 OUR NOSE WAS JUST COMING UP TO THE RWY. WE CONTINUED ACROSS AND HELD SHORT OF RWY 35. I AM ENCLOSING AN ARPT DIAGRAM FOR YOUR REVIEW. IN DISCUSSING THE EVENT LATER; THE PF AND MYSELF NEVER SAW A RWY 4/22 SIGN. THE CTLR SAID THERE IS A BOLD HOLD SHORT LINE; WHICH THERE PROBABLY IS; BUT BECAUSE WE SAW THE RWY 17/35 RWY SIGN WE BOTH ASSUMED THAT WAS FOR THAT RWY. IN RETROSPECT; IF I (PNF) HAD BEEN FOLLOWING THE TAXI DIAGRAM AS CLOSELY AS I SHOULD HAVE; I WOULD HAVE KNOWN THAT WE WERE PARALLELING RWY 35/17; AND THAT WE WOULD VERY SHORTLY BE COMING TO RWY 4/22. I WAS DISTRACTED WITH SOME COCKPIT ITEMS AND AT THAT TIME AND SHOULD NOT HAVE BEEN; IN THAT BUSY AREA. HOWEVER; THE PF WAS LOOKING OUTSIDE AT ALL TIMES AND WAS NOT SIMILARLY DISTRACTED.