Just my $0.02 worth. I just tried the plat w9dp plugs in my 84 500SEL and it was a big mistake. Ran terrible. With no other changes, put in NGK BP5ES (?) plugs and the car runs great again.
I don't know about the +4 but I don't think I will try them. The w9dc or w9dco or the NGK work so why take a chance again.
84 500 SEL (307,xxx miles)