FHA loans will consider rental income to some degree as long as it is documented. So that's no reason for making a decision one way or another.
I always prefer more doors to less so I would prefer the 5 versus the 4. But if you can get a very low money down loan on the 4 versus the 20 percent down on the 5 that would obviously favor the 4 unit. You have to run the numbers and see how things fall out. I've even seen folks buy duplexes and split them later and sell one half the duplex for the price of what they paid for the whole thing.
If you have the 20% down and don't mind putting it into the building, the 5 unit might make sense. As a first-timer I still think the two duplexes probably makes the most sense for you. You can put a very low downpayment down and have adequate reserves. There are no money down loans available from the USDA if it is a very rural area.
Good places to look for loans: credit unions (especially if you are a member), small and midsize banks, mortgage brokers.
Washington State is not considered particularly landlord friendly.
