I worked for a financial services company a few years back. We purchased private mortgage notes/land contracts, etc. from the note holders if they needed liquidity, etc. Like JG Wentworth but for private note holders.
A few things:
-4% is very low for a private mortgage arrangement, outside of a family member making a sweetheart deal to another family member. 7 - 8% was the norm, sometimes rates were as high as 15%. People in this country REALLY want to be homeowners.
-2 years is a pretty short balloon for a residential property, it usually 5 or 10, but not uncommon. 2 years is common for private commercial notes. Depending on what state you are in, if you can't pay the balloon and you end up in front of a judge, if you've made consistent monthly payments on-time and in-full, the judge will likely force the note holder to extend the balloon a few more years and allow you to keep making monthly payments. You don't want to be in that situation though.
-Is the seller using a note servicing company or collecting the payments and managing the escrow accounts themselves? It doesn't really make a difference, but you need to be diligent in recording and saving proof of every payment you make.
-Are there any other underlying mortgages/liens on the property? I assume you looked into all of this already before closing.
I dealt with the note holders themselves more than people in your situation but I was very familiar with this type of arrangement for a long time. I sure do hope you can find something lower than 4% in two years.