The problem with half stepping a drive will potentially be resonance and stall when the stepper is driven at certain speeds.
I would recommend you carry out some tests at quite a few speeds, you can write an incrementing feedrate macro that increases the speed of the 4th axis so you can test for any resonance issues. The key elements is all cnc applications tends to be acceleration and tuning the drives not just for maximum velocity (g00 rapid motion), but the acceleration setting which also requires you to shunt the axis not just back an forth but forwards in stages....
Basically spend as much time as possible performance testing at various feedrate to check nothing stall at any point also note that adding a cutting tool (non plasma or laser) will add some cutting force or resistance to the motion... making it more inclined to stall.... hence the basic rule to thumb tends to be, find the stall points, and choose a setting of about 60% of the peak stall and stall acceleration.... but each to their own... depends what you are cutting and with what.
Also with a 4th axis you may be just incrementing around the axis or 3d cutting (simultaneous) blended x, Y, z and a... b ... c axis motion...
Back to the start of my waffle... you may want to consider 1/4 or 1/8th ustep settings if you can... but no higher than 10usteps.. only a good set of testing will tell you and demonstrate any improvement with a flick of the dip switches