in a 4x4 i would say max 4 plants in 5 gal buckets if you want to grow them to any decent size. the only advantage in growing multiple plants in my opinion is you can grow different strains or select the best pheno from a batch of the same seeds. o dont use a mother plant so its more economical with the price of seeds to just grow one to a time. i enjoy ScROG when i can and 1 plant just works for me. like sp33 said its all a matter of personal opinion, if you only want to veg for 4 weeks then i would say go with 4x 5 gal buckets. if you dont mind vegging for 6-8 weeks 1 plant is all you need.
theres also the factor of having issues. if you have a problem with one plant it can be catastrophic or a casualty depending on how many you have. one dies and you have 3 left then its water under the bridge, one dies and its your only one you stuck starting over.
i guess theres pro's and con's to to each way..im pretty nuetral but when i started growing i was under the illusion that more plants = more yeild and thats simply incorrect, you can get a better yeild on growing only one and properly training it. my 2¢ im by far an expert though and still learning as i go. I actually am encountering a issue right now with my one plantt and will be asking the community about it shortly.