I sold 4.2232 shares of stock between Sept-Oct 2016 but I did not receive Form 3922, I just received a letter. What info do I need to input?

I'm going to assume that the stock you sold was stock acquired via an ESPP.  I believe that you should get a Form 3922 but if the "letter" has the same information as a Form 3922

https://www.irs.gov/pub/irs-pdf/f3922.pdf

then that's fine. 

I'd think you will also get a 1099-B reporting the sale.  It might not be until mid-Febrary, or later.

You'll enter the sale using the "Stocks, Mutual Funds, Bonds, Other" interview.

After entering information off the 1099-B you'll ask TurboTax to "guide you."  You'll tell TurboTax that you're selling stock acquired via an ESPP and then follow the interview along.  You'll be asked to enter the information that Form 3922 (or your letter) has on it.

Tom Young