First of all, if you're considering a true legitimate prop firm and not some sub-llc, then most likely they're a registered broker-dealer and they're getting 6:1 leverage from their clearing firm not 4:1 like some normal retail account. Second of all, most prop firms exist because they have tons and tons of their own capital to begin with, and they're trying to find creative ways to get the highest low-risk return on their money that they can get.... hence they start a prop firm and charge slightly higher commissions in exchange for letting you use their capital to finance your trades.
Take bright trading for example, I think they have $10 million of their own money sitting over at Goldman Sachs in addition to all the money of all their trader's capital deposits. That's a minimum of $60 million of buying power... I doubt they're too concerned about running out of buying power.
Take ECHO for another example... unless something has changed recently, they're owned by Merrill Lynch and thus Bank of America. I doubt they're concerned about running out of capital or buying power either.
Now if you're at some small tiny prop firm, or a sub-llc of another prop firm, then yah, its possible to run out of buying power. But who in their right mind would trust them with their money (although I know lots do... I just don't understand it).