Question about an underground splice

Folks, We're replacing a 600 square foot shed/barn. The old one had 220 supplied to it, from the house, via a 150 foot-long three-wire underground 4 gauge cable in a PVC tube. The old feed went into the building in a PVC tube that runs about 5' up the side of the building. The wire is aluminum. The new building will be located about 6' away from the old one. This means I need to do something to "stretch" the wiring. I'd like to (somehow) bury a splice from the old Al wire to a run of new Cu, and run the new Cu in PVC to the new building. We only need 30 amp 220 service so the 4 gauge should be fine. We will have a building permit and inspections. Questions: - How can I do the Al to Cu splice in a manner that's code compliant and underground? - Should the splice be in a box of some sort? What sort? - The feed to the building will be in a PVC pipe running vertically up the side of the building. My amateur reading of the code seems to suggest this needs to be 8' high before entering the building. That seems odd. How high must it be before the pipe enters the building? - My on-line reading suggests that we should have a 4-wire run from the house to the barn (hot-hot-neutral-ground). Is this a code requirement? I'd hate to have to re-run the entire cable.