The construction of a speaker like the 4-PI can be as simple or complicated as you want it to be.In its simpliest build, all you'll need is a drill, jigsaw, lots of clamps, and supplies such as glue and screws. Bring the cutting diagram for a sheet of 4x8 MDF or ply to Home Depot and have them cut all the panels to size for you. You may have to pay a couple of dollars for this cutting service, but well worth it if you don't have a table saw. At home, use the drill and jigsaw to cut all the holes for the driver, horn, input cup and port. Cut some braces to size and glue everything up with butt joints. That's all there is to it.
Joinery can be made stronger with biscuits, dowels, rabbets, dados, mitres or cleats but unless you plan on touring with these or have go-go dancers on top of them, the butt joints are plenty strong. However, the more complicated joinery can be neater, more air tight and make it easier to align the panels during glue-up.
"Upgrades" can include window-braces going in one, two, or all three planes. Flush mounting the driver and horn also improves appearence, but you'll need the use of a router, circle jigs, rabbetting cutters and template guides (for horn flushmounting).
"Difficulty" also depends on how you want to finish the cabinet. Paint, veneer, panelling, carpet? Trims and moldings can add a nice touch, as can roundover or bevelled corners.
Gar.