Welcome to the site, slingshot. 4 ga. may be adequate, but I'm not sure of your alternator set up, what your starter draws or how long the run from the battery to it will be. Without at least that much I don't think you can make a safe assessment.
Here's a calculator for determining the safe gauge.
http://beta.circuitwizard.bluesea.com/
Their entire site is good electrical reference.
Also consider checking out or buying Nigel Calder's "Boatowner's Mechanical and Electrical Manual". Probably the best $33 I have ever spent. Good luck! Where did you find your Seafari? and post some pics!