Just wanted to check if anyone has an internet source to ordering some....4 0z fiberglass seam tape....

Gordon-
I have purchased and used (though not on a kayak seam) some lighter glass tape. I've found that the particular tape I bought was quite difficult to wet out, as the combination of finer 'threads' and tighter weave made it more difficult for epoxy to penetrate. So I wouldn't recommend that tape- I forget the source, but it was either TheCompositeStore or Soller - both great companies to deal with IMO. That tape might work better with a less viscous epoxy than WEST.

On all the woven tapes, I've found that there's a definite 'ridge' along the tape edges after the epoxy cures.

On my current build, I used bias-cut strips of 4oz cloth on the outside seam. Those strips conform well to the hull/deck shape, and I could make them a bit wider so there was more width for sanding and 'feathering'. No ridge at the edges.
And, unlike 'straight-cut' strips, with bias-cut strips there's little ravelling at the edges so there weren't dangling strands to clean up or tangle in the epoxy brush.
With bias-cut fabric, you can easily stretch the strip, so youneed to be a bit light-handed when dealing with the strips.
Also, a rotary cutter is the tool of choice for cutting glass strips, IMO.

You could certainly bias-cut 6oz strips from any leftovers you have on hand, and IMO they would be an improvement over the woven tape. Worth an experiment, IMO.