I'd question that you can buy 40ksi in most of the US, as several have previously said. Even rebar that's labled as 40ksi probably still meets the requirements for 60ksi.
I conducted testing on mockups of old bridges. We had to order a special heat of 40ksi steel from the mill (only the stirrups were 40ksi) as no suppliers or mills in the whole state had 40ksi steel. In our material testing, most of the "40 ksi" steel yielded at ~55ksi or so, not too far below 60ksi steel.
I'd be interested if someone has actually seen a recent heat ticket for 40ksi steel and what it's actually yielded at. I think the stuff at Lowes/Home Depot is whatever is cheapest and most regionally available, so in most of the US, 60ksi (I doubt they have a heat ticket to prove it though).
There is still quite a bit of A615 grade 40 produced - more than I thought. Availability is very regional, and frequently you may be getting 60 ksi when you spec grade 40. It is not a true grade substitution if it still meets grade 40 ductility (ductility is the difference between the grades, other than minimum strength, and there is no upper limit on strength in A615.)
Weldability is only assured if the bar is specified ASTM A706 (in Canada it's Type W.) If I recall correctly, there once was a "weldability supplement" that certified A615 as weldable, but the current spec doesn't have one - it is all about carbon equivalent content.
If large post-yield ductility is really a primary concern (it seldom is except in seismic design), you can specify ASTM A706 Grade 60 or 80, which has higher ductility (12-14%) than A615 grade 60 (9%). In these specs, the ductility is dependent on grade and size of the bar. Stainless rebar ductility is a minimum of 20%.
ICF and swimming pool folks like lower yield strength so they can bend it in the field, often by hand. Since to bend it, you have to yield it, they like the low grade materials. This is fine for pools and artistic works. As a designer, you really should design buildings to avoid the need for that, since few field personnel will use the right bend radius, and tighter bends can not only damage the bar, but the CRSI/ACI bends assure that the concrete inside the bend does not crush and cause a bar fracture under heavy stresses. This is the failure mechanism of tighter bends in concrete. We are trying to educate ICF builders about ordering fabricated bar to make the work go faster and safer. The premium to have it fabricated is very small, and is much less expensive than field labor to bend the bar. I hand bent bar on my first ICF project, and it is not the way to go. Detail corner bars and minimum-bend, single leg stirrups to go over openings, and have them fabricated. Many fabrication shops use automatic benders that can knock out a house full of cut to length and fabricated bars very quickly.
The latest data suggests that A615 grade 60 averages 70 ksi, nominal; A615 grade 75 averages a little over 80 ksi; and A706 grade 60 averages under 70 ksi. I do not have figures for grade 40 handy, but most mills do not produce it.
The bar sold in retail centers may be non-spec, ungraded bar, so be careful with that.
