Read up on T-45 mild steel. It approaches the strength of chr-mo, but weighs more like Ti. F1 uses for steel applications where Ti is either too expensive or not strong enough.
The T-45 welds like mild steel and from everything I've researched, does not need to be stressed relieved like 4130. If you really want to get fancy, the French make a stronger/lighter version of T-45, but I forget the name of it.
I researched this quite a bit because I considered building a single cylider, self designed, naked bike out of the stuff. The only problem is that I could never find it in the US. It's mostly in the UK, DE or FR. It wouldn't cost too much to have enough shiped to build a bike. But a car, on the other hand...
T45 is a bit stronger than 4130 but that is it (4130 is 650 n/mm2 UTS, while T45 is 700, but yield is 620 vs. 520 for 4130)... it is steel after all and not lighter by any means (weight/volume..)
the wonder steel youare talking about is probably 15CDV6 / 1.7734.5 which is 980 N/mm2 with 790 N/mm2 yield... nice stuff, if you can afford the price - approx 50 euros per metre of 30x1.5 mm seamless tube.... this is what WRC cages are made of nowdays...