There is only one file type that uses the .WK4 extension:
Lotus 4 Worksheet
Spreadsheet created by IBM Lotus 1-2-3 (v.4), a spreadsheet program included with Lotus SmartSuite; stores data in cells that are organized into rows and columns; each cell can store static data or a formula, which automatically updates the value displayed in the cell.
NOTE: Previous versions of Microsoft Excel could import WK4 files, but Excel 2007 and later does not support the WK4 format.
Updated: April 27, 2011