Assuming lower mast is aluminum 4 NPS sch. 40: its OD is 4 1/2" with an ID a hair (0.026") over 4".

Nice dimensions for laying up a wooden top-mast using 2x4s. Lap 4 lengths, edge-to-face in a circular pattern, to make a 5" box, leaving a 2" square hole in the center. Add any desired taper by either taper ripping an edge of each of the four before gluing, or by merely marking out the corresponding skews and laying-up to these lines for gluing.

Leave sufficient hole at the top to pass a lightning conductor and whatever other wires are desired. Obviously a 3" square at the top will close the hole, so if this or more taper is wanted you would have to cheat-out the initial lay-up and correct it on the OD later.

After glue sets, trim to a box, octagon, 16-gon, etc finishing to round using your favorite power or hand tools. (Remove all of the temporary lay-up screws first! My hard-knocks lesson offered.)

Then go back to the bottom and form the cylindrical plug to the lower mast ID, leaving an OD matching shoulder above. My comfort-zone length is 4-diameters long as a minimum, but others may be comfortable with less. If you accept this then 10% bury would only come into play for longer spar extensions, say exceeding 13' or so for your size.

With a 4'± neat finished length this should be an easy task.

Pax tecum,

Michael

Well, you have the distinct advantage of living in a place that grows excellent spar timber.  You could probably simply buy a 4ft 6in piece of spruce and shape it to fit, making a 10% plug at the end of it to fit into your tube.  You could let in a wire for it, by routing a groove and setting in the wire, before epoxying and glassing the mast.  If you want it hollow, your easiest bet would be to buy some Douglas fir/Oregan and build a box section, tapered mast with plenty of wood to enable you generously to round off the shoulders to minimise chafe. 

I'm sure the more adept wood workers will be able to offer you other suggestions.