Hello Jason,
I too bought a C4 recently (from Axminster) and have been slowly going round it to fettle it in a bit. I also am getting a binding point at each end of the top slide travel - you are not alone in this it seems.
It arrived "workshop adjusted" and to be fair, was pretty good, apart from the red gooey snot stuff adhering to some exposed bits.
I have changed all of the jib adjusting screws (slotted ends!!) - which seemed to be made of putty, with high tensile socket/allen head grub screws.
I took each jib out and figure eighted it on wet-n-dry taped to a surplus, large bathroom tile and then linished to a shiny finish. Made a huge amount of difference to the movement of all of the slides and got rid of a tiny amount of slop which was apparent in the main cross slide.
The main jib strip on the rear of the saddle was gummed up with powder coat, and would not come free at first, plus you have to be a contortionist to get at it.
The strip on the top (compound) slide was flat, but slightly bowed along its length - very odd. I too am getting a smooth travel in the middle, but binding up at the ends of travel - even after fettling.
I think I shall have to make a new jib strip for this slide and see if that helps matters.
One other little problem I encountered concerned the main leadscrew.
I "stalled" the motor when taking a power feed cut on some really awful (scrap) steel bar of unknown provenance. The electronics shut it down and I re-set the speed, no problems. However, on re-engaging the clasp nuts, although the motor ran quite happily, the carriage stayed put. I noticed that the leadscrew was projecting beyond the end pillow block on the front of the bed end (right hand end). Turns out the main leadscrew is held into the drive train at the headstock end by a fairly chunky collar, nipped up by two clench bolts on a flat on the inboard end of the leadscrew. If these are not tightened up properly, the leadscrew can pull out under power feed if things get heavy going. It is quite easy to do these up properly, after removing the front of the motor control panel, and, touch wood, no further problems - even pushing the motor quite hard.
I am finding the two saddle locking pads - on the top of the apron, don't really grip tight enough - has anyone done a good saddle lock mod?
I would be interested to hear how it goes for you and if Steve can give us his views.
Best wishes
Peter