That might not be a bad idea, since the HTLA and the SS are of quite different chemistries. But, the things I mentioned above would still apply. To be honest, you should be able to weld the 4340 to the 304L without any problems with porosity. I mean, low alloyed steels are welded to SS every day without problems. If you do decide to butter both edges with 309, do not use that preheat when you start welding the 309 to 309. Again, I would recommend the 98/2 as your gas shielding.
Chuck