Brad,
I agree with Michael...you should account for vacancy/turnover costs in your calculation. There will likely be more turnover in a multifamily than you have had in your SFR's. And I would also suggest you run the numbers accounting for use of property management and hiring everything out...maybe you will do it all yourself now. But what if you grow your portfolio and need PM help, you move away, etc. At least you will have an idea of how that will affect your yield and ability to service the debt. And when you sell the property, other buyers will include that in their calculations of your property's value to them.
Also, you might/should be able to get longer term financing on a 4 plex. If the numbers look compelling I would do it on a 5 year note if that is all I could get, but it sure would be nice to secure some long term $ while rates are so low. Find an online mortgage calculator and run your mortgage numbers using your 5 year balance and play around with projected higher interest rates when you have to refinance. A few points higher will change your cash flow situation! Furthermore, I would be cautious with 100% financing, especially if you are in an area with historically flat/low appreciation. You want this investment to make your life better and move you along the road to financial freedom, not the converse. You should run the numbers with a variety of scenarios- consider your cash reserves, job situation, risk tolerance, 5 year goals, worst case scenario, all that. Lots of people finance cars of greater value than your LOC note, and that non deductible interest on a wasting asset! So I think that the 100% financing question needs to be framed by your situation as a whole.
Don't forget that having children will change your cash flow and 'free time'! Congratulations on the pending adoption. I hope that goes smoothly for you.
All the best,
John