$4k gets you anywhere in the world, and maybe half the western world is reachable with $1k or less. You're not going to spend a substantial portion of $80k on flights unless you go home and back every weekend.
It's not the end of the world, and the typical YC company will be resilient enough to survive whatever. However it does add up fast (40k easily), it's not something you can avoid, like say buy a less shiny laptop, and reduces your runway in a meaningful way.
If you are worried about the number of times you can enter on a visa waiver, don't - I have entered the USA 50+ times on a waiver and the guys at immigration ask me how I am going and wave me on each time.
This was even more fun before they closed 'The Mexican Loophole', where you could drop out to Mexico for the weekend and get a new visa on the way in. I had a lot of fun weekends in Mexico before they shut that down. You now need to leave the North American continent (they included Central America when people started flying to Belize to get new US entry visa's)
Most international YC companies end up moving to the US, and they sort out visa issues after demo day as it takes too much time during YC.
http://travel.state.gov/visa/temp/without/without_1990.html#...
Sadly, my country is not in the list, so I have to pay up.
The list of countries that you can get a waiver from is a lot longer now than it was when I started doing it. The USA realized that they depends on business tourism/migration, so they have been steadily expanding that list of countries and the scope of the waiver program.
So make sure you check back regularly.
I know of two people who did something like that and it went wrong. If you are not lucky you get the "wrong" immigration officer and then you won't get into the US anytime soon.
Immigration costs close to $10-15K if it's a real move. Without a US credit history, you're going to have to put down a lot of deposits just to get basic things done like getting a credit card.