I can see what you want to do, but am concerned at making an 8m mast with only 4" diameter. I think it will bend, and then break! At 8m, with rigging only at the top, the diameter needs to be 5" to achieve sufficient stiffness. The mast can be made slightly stiffer by making a snug mast case that holds the mastfoot firmly. This would double the stiffness (Euhler's buckling load), but I don't know if it would be enough.
Can you not buy a wood that is reasonably light weight so you can make a wooden mast? Timber weight should be approx. 30lbs/cuft - 500kg/cum. If the wood is a bit heavier than that you could make the wall thickness thinner. Using the 4" aluminium tube with two 1.25m long (0.25m into the tube) inserts will also get heavy, particularly if you use solid timber.