When you say periodically, do you mean a few years, months, decades? I ask because they are about a year old with only 10k on them.
Few years. I've heard 5 to 7 thrown out, but I'd imagine that mileage is more of a factor than age.
FWIW I did notice, as well as others, a big difference in stopping power and brake peddle firmness.
Of course.
But not only are you replacing old lines (10+ years, likely) with new, you're also likely doing a very good flush so you have no air and new fluid.
I noticed a big difference in stopping power and brake peddle firmness just with a flush of the fluid, so it doesn't surprise me in the least that you would see an improvement with new lines (any new lines) on top of that.
Won't rubber hoses dry out, crack and need to be replaced as well? Or is the point that you can see when they deteriorate and you can replace.
Keep in mind that stainless lines
are rubber lines, just with a stainless braid over them (and usually teflon between the line and braid).
So stainless lines face all the problems of rubber lines, just they have the braid which theoretically performs two functions. One, it protects from debris damage, and two as the rubber ages and has a tendency to swell, prevents that from happening.
The problem with stainless is that if the braid is flexed repeatedly, it can (and does) break.
Hopefully it breaks on the outside, where strands do no damage (at least to the line, to your hand when you grab them is another story). Just as likely it breaks on the inside, where the lines will puncture the rubber and cause pinhole leaks that will be difficult/impossible to track down (the fluid flows along the braiding and can spread itself out over a quite large area, meaning you may not see it dripping/leaking at all).
So, yeah, the point is that you cannot visually inspect the line. Just because the stainless steel braid is broken on the outside doesn't mean it's broken on the inside, and just because it's perfect on the outside doesn't mean it hasn't broken (and punctured the line) on the inside.
I've had dishwasher, sink, and toilet braided lines that chewed up my hand due to broken stainless braids (nothing worse than grabbing a line and getting stabbed a dozen times by extremely sharp ends), and they have a fraction of the flex/movement that brake lines on a vehicle do (hot/cold expansion and contraction, and movement as the line is pressurized).
Slee and many of the other stainless braided brake lines out there are teflon lined hoses are designed not to swell hence the performance aspect of them, so unless you physically damage a line they should last longer than OEM hoses. A visual inspection should show leaks and damage though.
The swelling of lines is something of a red herring. The rubber lines, if in reasonable shape, are more than adequate to withstand the pressure and not swell a significant amount (they will swell slightly, but so slight it won't even be noticeable). Very old rubber lines can and do swell, but tons of 80's are running around with original lines and no problems.
If you're running with an extremely high pressure braking system (think race cars), then the stainless lines might make a difference. But we're not, 80's have a pretty standard braking pressure.
I replaced the rear line from the frame to the axle (with a longer OEM one), and found that the original line (16 years and 160k miles old) was in virtually perfect shape. I found no evidence of cracking, swelling, bubbling, or any other damage.
And that there is the problem with a stainless steel braided line. I could look at my 16 year old line and tell with a quick inspection that it was in nearly perfect shape. It's impossible to do the same with any stainless steel braided line...you can't see the hose so it's impossible to do any sort of meaningful inspection on it.
) 
) If you're using the same shocks that go with the 4" lift, then there's absolutely no reason to go longer. Also, I know the front hard line spirals can be extended, I don't remember what the rear looks like, but that might cover you for some extension.
