If cells D4:D53 Is Not Null then lock. Else leave unlocked.

Hi,

I have a column A. The data in this column is auto populated from an If statement.

The issue is after a few minutes the data can change, which changes the data in the cell.

I need to lock the cell as soon as the cell has changed from empty to a value. The cells change from Null to having a value at a rate of about 1 every 10 mins.
So like D4 will get a value then 10 minutes later D5 will get a value, I need the cell to keep the first value it gets.

Please note the value is not manually entered but comes from and If statment.

basically i need:

If cells D4:D53 IsNotNull then lock. Else leave unlocked.

I have been researching for hours and tried many different solutions but can not get anything to work. Please help.

Any help is greatly appreciated.