I'm not sure where you're getting your info from. Weld on #4 and #16 both require a couple of days to fully cure, sometimes more depending on the application and environment. Both harden beyond workability in a matter of minutes. Weld on #16 is excellent for seams if you can apply it cleanly, but it does tend to goop and string. Using weld on #4 for seams works well but your seams have to be PERFECT, and I mean that. The last tank I built with 3/4 acrylic and weld on #4 I sanded all the seams with an aluminum block and 100 grit sandpaper before joining.
Google the "pin method" of joining acrylic <- I've had by far the best results that way. The first sump I made with 1/2" acrylic failed at one of the seams because I bought into the "just put the seams together and apply weld on #4 so it wicks in".