if you are surfing only in decemeber, i would go with a 5. i wear a 5/4/3 all winter. i went out last week in it and was sweating. it all depends where you surf, how long you will be in the water and how far into the winter you plan on surfing
different brands size there suits differnetly . I would sugest going to the website of the suit you are interested and look at there size chart then go to a good surfshop and try them on . Try a few to see what fits you best. I good fitting suit should be snug but not the the point where is restrics you movement