Well, I'll tell ya Al, personally, what anybody feels comfortable with is the line size they should use. And you are spot on with the 4lb and all the if's....
For years, all through the 70's, I fished nothing but 4lb on my spinning rod and 10lb on my baitcaster. The 4lb was mainly because as a youngster growing up, we went crappie fishing at the lake most every weekend and I was just plunking around the creeks during the week. Same with the baitcaster. We would bass fish the lakes in late evening or do some cat and carp fishing and 10lb was the most versatile line to use that would also hold a big fish.
As I got older and fishing more by myself and having to purchase my own equipment, I made some changes. Instead of having one lite spinning rod, I had two. One with 4lb and on the other I strung with 6lb to use mainly for rivers and ponds. Then later on, I discovered Crystal Fireline and haven't looked back since. Sure it is expensive on the top end but, wow the savings on the bottom end are tremendous! I used to use a minimum of 3-4 spools (reel spools) of mono a year due to twist, fraying and just plain wearing out. Now I spool about a 75 yrds a year of Fireline with mono backing about once every other year.
On the baitcaster, I started buying the larger sized mono for two reasons. First, I am cheap, but I will only use a certain brand of line and for me that is Stren. I always keep an eye out at Wally's for the clearance sales. I bought a buttload of 14 lb and 17lb Stren for $1 a spool one year. Still have some of that stuff. I liked it for two reasons. One it was cheap. Two when floating by some snags, and I got snagged, it wouldn't snap my line and depending on just how swift it was, it would hold the canoe. But I am downsizing back to 10lb this year.