Yes, four is enough as a signature.
Thanks for the heads up. Four will be enough to promote your service(s) and/or sites.
Yes four is enough, also you should have an limit on the Size of sig link, they also some times looks spammy.
Current character limits for NB signatures:
1000 characters including BB Code Markup
500 characters excluding BB Code Markup
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bogart (1 September, 2009)