I already posted what size drill to use. Banana plugs are nominal 4mm diameter, and having measured all the binding posts in my KEF R500 and R200c speakers, I know they are 3.91 to 3.93 millimeters. Being a machinist by trade, I know what probably hole oversize would be, so a 3.9mm drill or in the ANSI "wire gauge" size drills, a No.24 drill.
Consider just drilling these, deburr (which brass doesn't tend to create anyhow) and try it. If you blow it completely you would still be buying new binding posts even if you never tried drilling them. Heck, if you're near me I'll do it for you. It's a two minute job.