Three and even four and probably five (!) tubes will work, but you will not have a preheat for the extra tube ends. You can leave this as is (as Philips Certalume series ballasts shamelessly do) but it will drop the lifetime even further!

Just do not force tubes into EOL and stop the thing when they form some dense sputtered 'socks'.

Just curious, even if they are the wrong lamps, why wouldn’t it trigger eol protection on such a “smart” ballast? I mean, I feel like it would get upset when it notices the lamps current start violently changing right?