I have another question whats the specialty of modern (super) wide angles and the old classic wides or legendary wides.
(actually had apo lanthar and Sironar-S, modern Plasmats, in mind.)
Mainly asking about new type vs old type.
The old wide angle lenses such as the Angulon (a "reverse" Dagor type) usually just covered their format, and needed to be stopped down for good sharpness over the entire frame. Newer lenses such as the Super Angulon allow for movements, and will work at a wider aperture.
A different way to look at 'equivalent FL' is to merely compare the FL to the frame dimension. Using the short dimension of the frame as the baseline, then 24mm FL on 135 format would be equivalent to 94mm FL on 4x5 format. After all, you fit the short dimension of the frame into the short dimension of any print size, so the print is the great format ratio equalizer!
Comparing the 'normal', 50mm FL is 2.1x the short dimension of 135 format, and 200mm FL is about 2.1x the short dimension of the 4x5 format. Or, to put it differently, 150mm FL is 1.6x the short dimension of 4x5, so you really need 38mm FL on 135 to frame the same amount of vertical area.
So in 'really wide angle' land, 75mm FL on 4x5 is like using 20mm on 135 format.
About the old types, agree about the need to stop ancient types down to get full coverage, but modern types also gain coverage on stopping down.
Disagree strongly about the old types' limited coverage. Consider, if you will, f/18 Protars, Ross' f/16 versions, f/14 Perigraphes and f/8 and f/9 Wide Angle Dagors. I have a tiny 45/9 CZJ Goerz Dagor that covers 2x3. It isn't cataloged, I have no idea what its intended format is.
Disagree too that some of the modern types have lots of coverage. My little 38/4.5 Biogon covers a hair more than its intended format; has to cover 80 mm, puts good image in an 84 mm circle (huge difference there, eh?) and the illumination vanishes at 86 mm.
IMO -- not everyone agrees -- the significant differences are that modern w/a types in general are faster than the ancients, therefore easier to focus; that the ones that take advantage of Roosinov's tilting pupil trick have more even illumination than the ancients; and that the modern ones don't have to be stopped as far down as the ancients for good results. On this last point, for example, Rodenstock says that Apo Grandagons are best used between f/8 and f/11.
As for limited coverage, as a general thing an 80 year old WA for say 8x10, such as the Wollensak series III, doesn't allow for movements of any degree. I did say "usually" as a qualifier when I used the Angulon (which in the 90mm flavor doesn't give much if any wiggle room on 4x5) as an example.
Most modern WA lenses do give better coverage than their vintage counterparts of the same focal length.
I didn't want to go into any real detail in an answer until the OP is up to speed on the very basics. ;)
Clem, we've been wrangling, to some extent, about points that really aren't relevant to you.
90 mm lenses are often used on 4x5. So are shorter ones. You probably won't know which short focal length(s) is(are) best for you until you try several.
These days it is hard to lose a lot of money on used LF lenses, although at the moment prices of older 90s with limited coverage (Angulons, Wolly Optars/Raptars) may be on the high side because of a small craze for a new hand-holdable 4x5 camera that's designed around them. If I were you, depending on budget, I'd get a 90/8 Super Angulon and use it. I have an early one in #00 shutter; it is in terrible condition, I use it as a paperweight. Hold out for a newer one in #0 shutter. Same optics, shutter that's much easier to live with.
There are other similar lenses from Fuji, Nikon, and Rodenstock. I just took a look at keh.com, who have two 90/8 Fujinons at quite reasonable prices for that sort of thing. I buy most of my gear through eBay but keh is safer to deal with than sellers on eBay.
How you like it will guide you to what you'll like best. I'm sorry, but imagination and trying to reason from what you like on smaller formats aren't as good guides as one would think. You really have to get a lens, nearly any lens, and try it out. If it doesn't suit, sell it.
The ancient lenses I've mentioned in this discussion are either hard to use (90/14 Perigraphe, dim and in barrel, can be stuffed in the front on a Ilex or Alphax #3 but ...) or cult items (90/18 Protar, dimmer, usually in barrel) and horribly expensive. They're good, but aren't for everyone.
Good luck, have fun, and remember that everything you do will be wrong,
E., I brought up f/14 Perigraphes and f/18 Protars because they have enormous coverage. Pre-WWI designs, too. There's more junk out there than the common crap we're all familiar with.