When I ordered mine from Fritz last fall, he had the following to say about the 4 oz version (Arctic 1000) versus the Ultra (BPL) model:
[the Ultra]"… weighs five ounces, not four. The reason for this is that I thought the .004" stainless I used for the firebox wall {in the Arctic 1000) might not be durable (I noticed some buckling of the thin sections between the secondary air holes). The .010" that I am using now in the firebox is the same thickness that I use in the regular Bushbuddy."
I love my Ultra model and have found the extra durability to be well worth the extra ounce.
Sorry for the double-response. It looks like Roy beat me to the punch!