Which command is used for tool change when selecting tool 4?

The command used for tool change in CNC programming typically follows a standard format, where 'M6' denotes the tool change command. The number following the 'T' specifies the tool being selected. In this scenario, choosing tool number 4 necessitates the use of 'T4'.

Therefore, the command 'M6T4' is the correct choice because it correctly combines the tool change command with the designation of the desired tool—tool 4. Understanding this format is crucial for anyone operating a CNC machine, as it ensures that the machine knows not only to change the tool but also which tool to engage next.

The other options either use the wrong format for the tool change or reference the incorrect tool numbers, making them unsuitable for this specific command context.