You don't need to be 4.5' deep in the yard or untraveled areas.
Our laterals are 3' deep here...frost line is 18" at the deepest. A BX could pull off 3' without a problem, probably as quickly as a mini-ex could.
Unless I'm missing something, a mini-ex would only be of benefit over a BX backhoe if you're trying to do something beyond a 5' radius of the back of the tractor (full range of 6' being more time-consuming for a BX BH to accomplish). Most "homeowner" jobs are within that 5' to 6' range. Many contractor jobs are beyond that range. And, of course, the mini-ex could reposition quicker and not as often (with the 360 turret), but that wouldn't be of much benefit on an in-line installation.
As for pipe, personally I wouldn't use PVC pipe. PEX is the best...
I'd use polyethelene...the black stuff on the spool. Silly pipe, they call it. Perfectly appropriate for an outdoor installation and very limited fittings.
I don't think PEX is an outdoor type of pipe. Don't see any benefit to spending the extra money on it. Where PEX really shines is when cutting/fitting inside, not out.
Very expensive. Works best if there are other obstacles that make open cut a more expensive option. Not really for the homeowner, though.
Most cable guys around here use the vibrating slicer thingy...what Ditch Witch calls a "plow". You can pull a conduit 2' underground with nothing more than a 1" wide disturbance at the surface. It just 'pulls' the pipe underground. But you need an exponentially larger machine to do 3'+ deep pulls.
I own a TSC Sub Soil Plow that could do 24" with a couple of passes and very minimal disturbance. I bury low voltage line with it all day long. VERY efficient. Of course, that's just over 1' deep.