| Number | Compiler Warning Message |
| C4000 | UNKNOWN WARNING Contact Microsoft Product Support Services Level 1 |
| The compiler detected an unknown error condition. | |
| Note the circumstances of the error and notify Microsoft Corporation by following the instructions in the Microsoft Product Assistance Request form at the back of one of your manuals. | |
| C4001 | nonstandard extension extension was used Levels 1, 2, and 4 |
| The given nonstandard language extension was used when the /Ze option was specified. | |
| In C, this is usually a level 4 warning, while in C++, this is always a level 2 warning. When compiling C with either the /f or /qc fast-compilation command-line options, a function pointer cast to a data type will produce a level 1 warning. | |
| If the /Za option has been specified, this condition generates a syntax error. | |
| C4002 | too many actual parameters for macro identifier Level 1 |
| The number of actual parameters specified with the given identifier was greater than the number of formal parameters given in the macro definition of the identifier. | |
| The additional actual parameters were collected but ignored during expansion of the macro. |