It has already been defined that for fly fishing ultralight means 3-weight or less, but for UL spin-fishing it does seem less well defined. Also, somewhat like fly fishing, the term ultralight seems to have changed over time. In the early ‘70s a 5-weight was considered ultralight for fly fishing. Four pound test was also considered ultralight for spin-fishing.

In that time period, I used to use a 5 ½ foot UL rod with a Mitchell 408 to fish successfully for coho salmon and steelhead, but it was overlined with 6-pound test. So some people even then would not have considered it truly ultralight, but I did. Nowadays 2-pound test, with a matching rod and reel, would still be considered ultralight. I’m not too sure about 4-pound test, but probably so, and like you say, it does make it easier not to lose as many lures.

To me it’s rather amusing to note that on another forum there are members who take a dim view of fly fishing with a rod for a 3-weight or less. However, if a UL spinning rod is built up from the same blank that’s quite acceptable and even desirable. To confuse matters even more now there are braided lines that are smaller in diameter than monofilament but even stronger. To sum it all up, I also don’t know what is or isn’t considered ultralight spinning gear, but would like to know what others have to say.

Generally the rod makers are going to mark them "Ultra light" or "UL" - sometimes the action is referred to as ultralight, but they really mean power rating. I've had UL rods that were slow action, UL power, and I've had (and favor) fast action, UL power rods. To me, UL rods are those that are designed for 6lb or less line, and that tend to flex at least to the middle or into the butt when fighting a fish. They'll protect lines as light as 1lb with the limberness.

I've got a book I've cherished since I was a teenager called "Advanced Tactics for Bass and Trout" by Jerry Gibbs, the former editor of Outdoor Life. In it there's a section on "Noodle Rods" - long (9 to 12 foot) whippy rods that guys in the Great Lakes trolled for Chinook Salmon and big trout with using 1 to 4lb test line - this goes back to the 70's / 80's and guys regularly landed 50lb salmon on them.

There's also a section in that book he talks about spoon fishing in low water conditions with ultra light rods for brown trout. Great read if you can find a copy. He covers both conventional and fly fishing tactics in that book, for both bass and trout (and they apply readily to other species as well.)

Of course Ultralight Rods and Ultralight Fishing don't necessarily mean the same. Ultralight rods talk about the light end of the tackle spectrum as a whole - whereas Ultralight Fishing is species dependent. I can take my heavy power fast action bass rod off-shore fishing, and I'd be going ultra light for the big boys like Marlin or Tuna or Grouper.

I've got another book called "Catching Big Fish on Light Fly Tackle" by Tom Wendelburg and he actually goes into what was mentioned by the OP - using a 4 weight rod to chase steelhead in the Great Lakes tributary streams. I'd say that would qualify as ultralight fishing, since steelhead are usually targetted with 8 to 10 weight single handers, or 6 to 8 weight spey rods.