Where do I report "net long-term capital gain" from box 4a on Schedule K-1

[RichardG]

I assume this K-1 is from a trust or estate.  You'll report that number as part of reporting the K-1 in TurboTax.  Please go to Federal Taxes>Wages & Income>Other Business Situations.  You'll find Box 4a as part of that interview (see screenshot below).