Ah, my bad (about both Steripen and the chlorine dioxide).
I normally just suck water through my Frontier Pro, but have tested it a few times as an "in line" gravity filter. There was no issue with water leakage at the end -- which leads me to think that maybe yours is damaged from use or just defective. If you like your setup, maybe it's worth buying a second Frontier Pro.
As for the 4.0 micron size Steripen "Fits All" prefilter -- no firsthand experience whatsoever, but you got me curious...
According to the REI write up, cryptos range in size from 2 to 5 microns. REI recommends filters with an absolute size of 0.2 - 0.3 micron to filter out crypto. Not a pathology expert, but I'm sure there are reasons why MSR and Katadyn filters are sized down to 0.2 and 0.3 micron respectively. I don't know if I would rely on the 4-micron prefilter alone to block protozoa.