http://www.supertool.com/StanleyBG/stan5.htmTake a look here. You'll have to scroll down to find the part on the Stanley scrub plane. I've got one of these girls, and it gets called into action when I have a rough slab to get into shape for smoothing. Often the slabs I work with are 10-12' long and up to 4" thick and can be 4' wide. There can be a lot of winding in one of these, and considerable curve lengthwise. A lot to remove to get one side flat enough to begin working the other side down to bet a piece with two parallel surfaces and consistent thickness.
There are new scrub planes out there, but why bother? I picked mine up from a guy who mostly sells to collectors--shudder--but this one was "ruined" by the owner having put his initials into it. Collectors don't like that sort of thing, but for a working tool, it matters not.
I suppose you could grind the iron on a #4 or similar to a considerable curve, but a #4 has a 2" iron. That's a BIG bite to take out of a plank. You'll work yourself to death purty quick. I think the scrub is a 1" iron--skinnier, anyway.
A scrub plane is pretty amazing. It looks impossibly small and meek, but it will really do the job. It's not a plane that many folks have a use for these days, however. But if you do think you need one, watch for a bargain. There are many out there with bad finishes and fairly dinged up, but they can be put back to work fairly easily. BTW, I bought mine for $30, cleaned and sharpened it, and just out of pure vanity, removed the knob and tote and sanded them down and refinished them with spar varnish. You can do the same.
Just try real hard not to look too long at the Lie-Nielsen catalog. The ol' Stanleys still have a lot of work left in 'em.