Supposely, the new Frontier 85 has improved performance over the original, too. Unfortunately, we have no way to know if any type of features are effective. Safecell or no Safecell, metal or plastic, this side impact technology vs. another one. Some sound good in theory, others only in marketing materials. Making matters worse, the pass/fail testing and research information we have is all but useless for the purpose of determining how safe a product is or allowing us to compare one product to another:-(
In any case, I'd also say go with the Frontier 85 for a 4-year old instead of a Boulevard 70. My 5-year old rides in a Frontier 85 and I have a Marathon 70 to use as well. I have no issues with him in the Marathon 70, I just wouldn't buy one for a 4 or 5 year old only to get maybe a year of use. Of course, you have a younger child who can use the Boulevard 70, so that may change the value end of things for your situation!
Incidentally, just feel how sturdy the shell of the Frontier feels when you get it. It's not flimsy or hollow plastic, it's strong, high density stuff. Sometimes, plastic is actually superior to metal in various applications, it all depends on the type of the plastic, the type of the metal and the quality of the design. If everything else was equal, having a high strength steel is stronger than plastic. Problem is, everything else can't be equal. You either have to use significantly less metal or accept that the product will weigh a lot more. In using that much less metal, it could be that plastic is just as strong or stronger for the intended purpose. As consumers, we just have no way to know without reliable independent crash tests that allow us to compare one model to another.