The rule of thumb is, "50x per inch of aperture", so 200x for a 100mm (4 inch) refractor is about right. You
may be able to get more, but the advertiser is being conservative and that's a lot better than other advertisers who claim ridiculous magnifications for small refractors, notwithstanding the commonly-accepted limit of 300x due to the atmosphere. But see this:
Where does the 50x per inch limit on magnification come from?