It's pretty common for some ARE programs to have 4 years of funding.
As far as getting an additional year of funding, It really depends on the school and how much money they have. My advice is to assume that getting an additional year of funding is not automatic so when you get to your fourth year, you can start asking professors if they need RAs and apply for fellowships.
From talking to people in econ, ARE and policy, many people fund their unfunded years through teaching gigs (sometimes at community colleges), student loans or personal funds.
With people now finishing in 6 years in econ, I assume that schools will start adjusting, but right now, that does not seem to be the case, at least for the programs (LRM) I am familiar with.