Turning 304 SS is quite different than 4140. It largely has to do with insert coatings. The correct coating is absolutely necessary to get good insert life with 304SS. The material will weld itself to the wrong coating and begin to pull pieces off the cutting edge, commonly known as the built up edge phenomena. So consult your tooling catalogues as to suitable coatings for turning the 304.
The insert shape also has a lot to do with good chip flow. If the chip is piling up on the tool, such as it often the case with VNMG, it will wreck the tool. The VNMG is a negative rake insert, and does not have a positive rake chipbreaker to promote good flow in stainless cutting.
I don't know of any insert that does a good job of short-chipping 304, because those that do, force the chip to break on the side of the insert and this causes crating on the side of the insert.
Personally, I like to use Kennametal/Hertel Fix_perfect style inserts for turning 304SS. This unusual shape of tool (eight sided on edge) will roll the chip into a smooth spiral "cable" that may break between 3 to 12" in length. But, nice to handle, and they will fall down in the chip pan. I usually rough at about .05 to .1" DOC and .005"/rev feedrate with this tool. Surface speed, I keep down to about 300 fpm, as the heat buildup at the tip can be extreme with stainless. I use lots of flood coolant, too.
Iscar Cutgrip will also do an excellent job in 304SS. These also roll the chip up into a spiral cable. The correct coating is still mandatory for good insert life.
Turning C4140, use CNMG's to rough and finish with. There's lots of good grades and coatings that will handle it, and this is an economical insert shape to buy. Use the VNMG sparingly, it cannot handle much "plunge and turn", and should only be used where a tiny profiling feature demands it.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)