I'm with bnoon. Never had a problem with +4 plugs or +2 plugs in the SVT. Whan I removed the +4 plugs after 30k, they were as normal as any other plug I've changed out. Chalk this one up to YMMV.
'98 SVT
KKM / TH fix / Mobil 1 / Synth tranny cocktail / JVC LX-30 HU / 10 JL sub / Alpine V12 amp / Panther 199 wheels / Kuhmo Ecsta 215/45/17 / Bosch +4 / Ford Motorsport wires