Rental investment properties and down payments

[Alex Romano]

I have watched a number of webinars recommending rental properties investment but I feel like I'm missing something..

Say I want to invest in a duplex for $200,000.  I use $40,000 (20%) from my savings as a down payment.  Ideally, the rooms are rented and I earn $200-$400/month in profit on this duplex.  The goal is to then invest in a fourplex (I'll estimate that the fourplex costs $400,000.)  Assuming that I don't have $80,000 for a down payment, do I purchase this next property with a down payment that's less than 20%  Is the best option to find an investor who is willing to work with me?  If that's the case, what do these arrangements/agreements typically look like? 

This is all new to me and I understand this is likely an amateur question.  If someone could please at least point me in the right direction towards a helpful resource even, I would appreciate it!

Thank you!