Two observatories one has Bortle 4 skies, the other Bortle 7. A third location is also Bortle 4.
Above 4” aperture all my scopes are long focus, F/7 - F/9. None have reducers.
That's still fine for stellar type objects unfiltered. Try M15 and M2, with and without I3. Then with and without Moon. The Moon will show the benefit of the long pass IR, even on stellar type objects, as the background gets higher. 891 is a good galaxy to test unfiltered from Bortle 4, but the Moon will kill that dust lane.
My 120mm is f/7.5. It's a great image scale for filtered nebulae, but it's noticably dimmer than my 80mm f/6 or 15" f/4.78 as paracorr'ed. Not a deal breaker, but a reducer would brighten than right up, at the cost of image scale. My 120 is still mostly visual, with only incidental NV use. The 80mm and the dob are my preferred setups for that, with a factor of four increase in focal length, and thus image scale, at comparable f/ratios. This is fundamentally an imaging technique, so f/ratio determines how bright the objects are, and aperture determines how resolved the objects are. Two f/7 scopes are as bright, regardless of aperture, and two 5" scopes are as resolved, regardless of brightness.
Filtered Propeller Nebula (PN) is a good comparison between the apertures. From my Bortle 5 yard and a 6nm filter, the 80mm sees the cloud that it's in, but not the PN. The 120mm can just barely see the PN, when there is excellent transparency. The 15" always see it, except when there is very bad transparency.
An Astronomik 642 longpass has a bandpass from around 640-850. It's a great filter for my skies at home without dimming stars too much. The h-alpha portions still get swamped by background. The Swan, Orion and Flame nebulae have lots of reflection portion in addition to h-alpha. Those work unfiltered from Bortle 5, and do much better with the 642. I like my Baader UHC-S for those. The background is darker than with the 642, but not as dark as with the narrow h-alpha. Many more stars get through. The UHC-S has broad bandpasses around h-beta/OIII out to Carbon lines, and a broad 35nm h-alpha pass. At my Bortle 2/3 sites, I prefer having UHC-S, 12nm, and 3nm in the slides. At home, the UHC-S just barely shows the Horsehead. That prefers a narrow pass. You may happen to have a similar foter on hand, with an h-alpha pass region, so give it a try if you do. I'd recommend a 6nm h-alpha to start off with for your Bortle 4 skies, then a 642.
Reducers, if you so choose, would leave resolution unchanged, but brighten the image at a lower image scale. A lower focal length scope at a comparable f/ratio is a perfectly acceptable alternative to reducing one scope. With extremely narrow h-alpha filtering, these don't need to be APOs. For unfiltered things, such as globs and galaxies, I prefer the dob anyway. Rather than the cost of a reducer, a used guide scope is a reasonable alternative for h-alpha filtering.