30V for a 4-foot wire should be more than enough, probably too much. When cutting foam, you want the wire hot enough to melt through the foam and that's it. If you're making any smoke, your wire is TOO HOT, so cut the voltage back.
The voltage required will depend on the wire you use. Many guys use stainless steel or NiChrome, but if you want to do it right, get some Rene` wire. It's thinner, stronger, made for higher temps and doesn't stretch anything near as much as the other wires. It's what I use. http://www.skykingrcproducts.com/acc.../renewire.html