If you make $50K a year, you shouldn't borrow $400K, even if you can find a lender who would offer it to you.
Even with eager potential roommates, your co-signer would have to front you around $200K for this to even been in the realm of risks that are worth considering.
author, "Diary of a Real Estate Rookie"
A good mortgage lender will not let you do an ARM loan nor an interest only. Interest only loans are for people that are expecting to cash in on something big in the near future.. are you? may be expecting a hefty inheritance? or other?