What kind of scanner antenna do you have? If it doesn't resonate at the low band frequencies, you can get away with having them closer without damaging the scanner. I have a scanner antenna about 4 feet from a 25W dual-band antenna with no issues. If the RX antenna doesn't resonate at the TX frequency, you can safely run higher TX power at the same distance.
With 100W TX power and 4ft of separation, I'd be concerned if the scanner antenna was designed to RX the TX frequency. But if not (e.g. low-band TX is 40MHz and scanner RX starts at 110MHz), you'll probably be OK. Unfortunately the only way to know for sure is to key the mic and see if you fry the scanner's RF input.
Regardless of damage concerns, don't expect to hear anything from the scanner (other than yourself) when you have any other radio in the vehicle keyed.