If you rarely play, four million is enough.
Most people who merch, well, they often play, but they also have around 50,000,000gp + (as stated above).
Obviously, for more profit, you need more beginning cash.
I'd prefer training, for quite some time, until you have perhaps...
20,000,000gp (20M). Around there, do simple things.
Merchant essence, logs, potions, armor, etc.
You'd be making perhaps, a one or two million gp profit per 2-5 days.
Average per day, is quite nice, no specific price though.
It's something you have to keep working at, obviously once you gain cash, your profits increase, etc. etc.