Before getting at your 4 questions, I want to say right here that "it always depends". Lame, I know, but it really depends on the niche that keyword is from. For example for a local business, getting 500 visits a month can be a lot (depending on the niche) while for a site in the weightloss niche this would be a long-tail keyword most people wouldn't even waste time to target.

The next thing is that just because you target a keyword with lots of searches, it won't mean nothing if you can't rank for it. So for some people 10k searches might be a lot (that they can barely manage to rank for) while others don't even get out of bed for 50k searches.

With that said:

1.) Generally, for the average marketer anything above 9k exact match searches a month is "a lot". Unless it's a golden nugget keyword with low competition, some people may not be able to rank for these kind of terms.

2.) Average is anything between 1.500-3.000 searches a month (as always, exact match).

3.) Anything below 1k searches a month can be considered low.

4.) This depends again. Just because you're in the top 4 for a keyword it won't guarantee that you will receive more traffic, especially not if the site of the original brand or company is ranking around you.

Apart from that, I'm sure you've seen the following graphic already but if not that should give you a good idea. I want to note these numbers should be used as guidelines, not as a fact. How much traffic you get won't only depend on your spot but also how you crafted your title/description so it get's a healthy clickthrough, your domain name etc.