What is the WK4 file format?
.wk4 files store data used by Lotus 1-2-3, a spreadsheet application included with the Lotus SmartSuite office suite. The format was introduced in 1993 with Lotus 1-2-3 Release 4 for Windows and remained in use through Release 5; it was later succeeded by the .123 format in the Millennium Edition. Lotus 1-2-3 itself was discontinued by IBM, with sales ending in 2013.
A spreadsheet is a type of document that organizes data using tables. Tables consist of rows and columns that create cells capable of holding various data - numbers, labels, or formulas. .wk4 improved on its predecessor .wk3 by introducing 3D multi-sheet workbooks, with each sheet supporting up to 8,192 rows and 256 columns. Spreadsheets are commonly used in business and finance contexts as they are well suited for organizing and calculating numbers.
Only older versions of Excel support this format - Excel packages up to and including MS Office 2003 can open .wk4 files. Versions from Excel 2007 onward dropped support entirely. Today, LibreOffice Calc and Gnumeric are the most reliable tools for opening and converting .wk4 files to modern formats such as .xlsx or .ods.
Security & safety
RISK: LOWWK4 is plain spreadsheet data with no macro/scripting payload of the kind found in modern Office files, so opening one is low risk. The real concern is data preservation: WK4 is obsolete and modern Excel can't read it, so important data should be converted to XLSX/ODS while LibreOffice and Gnumeric still ship the Lotus importers. Avoid sketchy "WK4 converter" download sites; use the official LibreOffice/Gnumeric. Lotus password-"sealed" files used weak protection and are not a security guarantee.
Format details
in a nutshellPrograms that open WK4 files
Windows3 apps
Gnumeric
Open-source Open the .wk4 - Gnumeric supports the whole Lotus family (WKS/WK1/WK3/WK4/.123). Good for tricky legacy files. 
LibreOffice Calc
Open-source File > Open the .wk4; Calc imports it directly. Then Save As to XLSX/ODS/CSV. The most reliable free opener. 
Corel Quattro Pro (WordPerfect Office)
Paid Open/import the WK4 in Quattro Pro, which historically reads Lotus formats. Paid suite; use only if already owned. 
macOS1 app
Linux2 apps
Technical details
deep spec| File structure | Flat record-stream of binary type-length-value (TLV) records from a BOF record to an EOF record |
| Magic bytes | 00 00 02 00 at offset 0 - BOF record type (0x0000), length (0x0002), followed by a 2-byte version word identifying WK4 |
| Byte order | Little-endian |
| MIME type | application/vnd.lotus-1-2-3 (also application/x-123) |
| Encoding | Binary (no text encoding; strings stored as length-prefixed byte sequences) |
| Compression | None - records are stored uncompressed in sequence |
| Encryption | Optional password protection using Lotus 1-2-3 sealed-file scheme; weak by modern cryptographic standards |
| Grid limit per sheet | 8,192 rows × 256 columns |
| Multi-sheet support | 3D workbooks with multiple named sheets - key advancement over the single-sheet WK3 format |
| Developer | Lotus Development Corporation; later maintained by IBM after the 1995 acquisition |
| Primary application | Lotus 1-2-3 Releases 4 and 5 for Windows |
| Succeeded by | .123 format introduced in Lotus 1-2-3 Millennium Edition (Release 9) |
| Excel compatibility | Supported in Excel 97 through Excel 2003; import capability removed from Excel 2007 and all later versions |
| Current openers | LibreOffice Calc, Gnumeric, and Corel Quattro Pro (WordPerfect Office) retain import support |
| Released | 1993 (Lotus 1-2-3 Release 4 for Windows) |
| Latest version | WK4 (Releases 4 and 5); succeeded by the .123 format in Millennium Edition |
| Specification | fileformats.archiveteam.org |
WK4 conversions
Community Q&A
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