Can I safely take 400mgs of zoloft (sertraline). I'm currently taking 200 - 300 mgs daily. My depression varies and today its bad?

Please don't up your. Dose dependent on how you feel one day. It takes quite a while for serotonin levels to increase - so upping your dose for a day will not make symptoms better. 400 mg of zoloft (sertraline) is well above the recommended dosage. You need to discuss this with your psychiatrist/ physician. See that you drink socially - that is definitely contraindicated if you take zoloft (sertraline).
No ; stop alcohol. 400 mg of zoloft is twice the recommended maximum dose. It is possible that you're getting an inadequate response due to alcohol. In my experience, ssri medications do not work well in the presence of any alcohol. If stopping all alcohol does not allow for a better response, then adding 2 mg of Abilify (aripiprazole) might make a big difference.
Probably not. Sorry to hear that your depression is not under the best control. However, antidepressants like zoloft (sertraline) do not work like painkillers as in taking an extra dose 1 day will help that day (if you notice any improvement that day, it is probably the placebo effect). It's not wise to keep varying your dose of zoloft (sertraline) from day to day or to take 400 mg. Talk to your doc about rx resistant depression.