I've read that traditional banks will offer around 4 mortgages, and then it gets much tougher, probably requiring new avenues such as portfolio lenders, but I wonder if we could even get those first 4 mortgages?

This means your buyer would only need to take out a traditional mortgage of $300K.Also, I believe (don't quote me) that in this scenario the buyer would be able to avoid paying PMI as the balance on the first lien would be less then 80% loan to value.  

We both have traditional incomes coming in. 

Depending on your market, you will likely get a higher price than if you do traditional wholesaling.  

If we're talking in the sense of "traditional," investing with traditional financing means then yes there is no more 10% down.However combining creative with traditional investing can often times allow you to put down 0% and even end up with cash back, cash flow, and a property.

This question is aimed particularly at seasoned investors near or at retirement age or investors who also are certified financial planners:Once you reach retirement age, what percentage of your portfolio reasonably should be in real estate vs. a traditional IRAs or other financial instruments? 

The benefits are that you have 1 policy for all of your rental properties, premiums are lower and there is better coverage because it is a commercial policy and covers more than traditional residential policy used by many investors.  

My understanding is it would be refinanced with a traditional lender?

Of course, within notes there are easier and more difficult approaches such as buying up performing notes from an investment provider (turn-key) to shopping for discounted, non-performing notes.In a perfect world, one would create a portfolio that has a nice balance of different properties and notes of various types in different markets to achieve diversity - as well as perhaps some traditional equities/funds, maybe some precious metals as a hedge, etc.A budget of $100K could most likely be placed into one or two key investments.  

The lease is a traditional lease agreement you would typically sign to rent an apartment.