In my experience it seems to depend on how fast you can type. It appears that the F4 key repeats the last second or two of actions so if your last keypress was a couple of seconds after the prior one then you will only get one character but if you blast out the word then F4 will give you multiple characters.

I have no idea on how to get Word to prolong the F4 'recording' of keystrokes but it may be configurable.

To get repeatable F4 behaviour I would copy the text and paste it once, then press F4 to repeat the paste action.

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Andrew Lockton
Chrysalis Design, Melbourne Australia