I was looking at the feasibility of brute-forcing your hash last night.
You use a 10 character base64 'junk', so even if I can predict the exact time, and guess the right preselected 0-9999 number, I still have to run an expected (64^10)/2 = half a million million million hashes to see if I'm right.
The entire bitcoin network currently hashes at 12 Thash/s. So if I could convince them all to help me win your 4 BTC wallet balance, it would take us about 13 hours to brute force one of your hashes (given that I already guessed the time and preselected number correctly).
So I think it's pretty safe to increase the time the user has to make his guess a little.
The bets will get bigger!
The balance is actually about 20BTC right now, with bets that win 20% being the max, but like you say the 4BTC is all that matters for brute forcing. If you ever lose a bet you'll be able to bet at least a little bit more the next time, unless someone has reserved some of the balance in between.
But, yeah, I just changed it to an hour. Should be good enough.