Shimano Tiagra 4700 10 speed lever only shifting 9 gears I have a Shimano Tiagra 4700 10 speed group set.

I can only get 9 gears working - the shifting is smooth with no jumps but one gear is missing, either at the top or bottom depending how I setup the rear mech. If I disconnect the cable from the rear mech and then hold the cable by hand I can feel the shifters click through 8 times - one click too few!!. So the problem seems to be in the shifter, as if there is something blocking the shifter from turning that last slot. 

Could it be a faulty shifter? Is it easy to take them apart to check?
By removing the cable from the derailleur and operating the shifter while applying tension to the cable by hand you have proved the shifter is the problem (unless the cable is hanging up somewhere, it's best to do this test holding the cable as close to the shifter as possible).

Sometimes shifters can get a bit of dried grease or dirt in them that prevents a pawl from catching on a ratchet. Unfortunately they cannot be disassembled as they are riveted together.

Soaking with WD40 or equivalent and/or blowing out with a compressed air hose can reportedly clear the contamination.