House wrap needs backing to apply smoothly. It will be wavy, and if you have wind, it could rip off before you apply the Hardie panels. The top of that 1x4 PVC trim, if thicker than the panels, will catch rainwater which will wick into the bottoms of the panels. If you use OSB under the panels, that problem won't be there, as the thickness will be greater than the trim. If The Hardie board calls for six inches of clearance, then you should use 1x6 trim. I have a cabin where some of the panels are too close to the ground and have been damaged along the bottom by rainwater splashing up onto them.
