Likely that it's not a matter of allowing an upgrade, but rather more likely the fact that it's not possible to upgrade the internals without losing money on the deal.
First and foremost, Eco-Tech Marine is a business.
If it cant make money, it wont remain in business.
They make a good product and have excellent customer service. I recently upgraded my Gen1's to Gen 3's. I'm happy with the upgrades and feel that the cost to upgrade was fair for what I got.
It's likely that upgrading Gen 3's to Gen 4's (if possible) would essentially entail stripping everything out of the Gen 3 and replacing it all with the Gen 4 internals.
If that is in fact the case, buying an upgrade that is very close to the price of buying a whole new fixture doesnt make sense financially to anyone.
I'll use my Gen 3's til they fall apart, and then I'll upgrade to whatever is available at the time. Hopefully, that will be an Eco-Tech product at that point, but if there is a better product at a better price, I'll look at it.
Eco-Tech's Customer service has been exemplary every time I've needed them which goes a LONG way towards future purchases from them, but like anything...they need to be continually improving and evolving their product line.
Which leads us full circle back to the Gen 4 Radion....