You're right, but the price of the crypto used for the bet, is also an issue, if it falls during the period, you'll get coins that worth nothing in 4 years.
There are multiple risks. Obviously the cryptocurrency exchange rate can go down, just like what happened during 2014-15 period. But a greater risk is the gambling site itself getting closed without any warning. I am active in this forum for almost 4 years and I have seen this happening in front of my own eyes. Many of the gambling sites, which existed in 2017 are no longer active. Most of them vanished all of a sudden, taking the customer funds with them.