Do you wish to make this change across all styles? There's a few ways you can go about doing this, but I'd read this entire post before trying anything.
You could backup your /forums/images/buttons/ directory (or just the images you will be replacing). Replace the vB4 postbit buttons with the vB3 ones (while using the vB4 names). You will also need to replace the hover ones as well. So for example, for reply you would upload 2 copies of the vB3 reply button. One named "reply_40b.png" and "reply_40b-hover.png"
OR if you don't worry much about space just copy the buttons directory, name it something else, then overwrite your images there. Then for your styles change the imgdir_button stylevar to the new folder...
The potential issue I see with doing this, is that the template and CSS will make your buttons look like this. For an example I just tried with the quote button:
Removing the text is simple enough, you would go to your postbit or postbit_legacy template and remove the {vb:rawphrase}'s for each button. So for reply you'd remove {vb:rawphrase reply} from the template. But once you do that, you'll need to play around with the CSS to get them to fit right....or completely move them somewhere else in the template.