Weldon 16 is a thicker, more syrupy version of Weld-on 4. I dont know if they surfaces they will bond are different or not however?
16 Will act as a 'gap filler' as well, but the portions where it is not a direct bond between the 2 surfaces and the 16 is just filling will not be nearly as strong(infact you can break it off with a fingernail).
Weld-on 4, when applied correctly, is amazingly strong! I have heard it should work for ABS as well.
If you can find a page with the different uses of each Weld-on, you should be able to find one for the job you need.
This site does not seem to indicate #16 will work with things other than acrylic however:
http://www.ellsworth.com/display/pro...&productid=804
Here we go... didnt really read through it much but this page might help:
http://www.sdplastics.com/weldon.html
Cheers,
Chris