As was mentioned above, brazing may be the best way to approach this, if the thickness of the wall is near the 2mm figure (or less), and depending on the price quotes you get for TIG welding.
If the 4mm figure is more accurate, and the alloy is correct, then TIG welding should be a quite reasonable proposition--for someone experienced.
I have done TIG welding before, and while I would not call it an art form, it is certainly more difficult than welding any other material you are likely to work with. It was difficult for me to tell when the Al was getting hot, as it really doesn't change color the way steel does, and would just 'puddle up' all of a sudden, like solder.
I never got to MIG weld Al, since the wirefeed welder at (high) school was setup for welding steel, and I've never really had the incentive to hunt down another one to play with.
Brazing on the other hand, is pretty darned easy, is possible on basically any metal (as far as I know), moderately strong, and will add little weight to your javelin. Plus, you can do it with a few dollars worth of materials, and a cheap torch.
