Netflix 4k is using h.265 as the codec.(otherwise known as HEVC). Most embedded systems need hardware support of a particular codec since they lack enough processing power to do the decoding in software. To my knowledge, hardware support of h.265 in SoC (system on chip) implementations is exceedingly recent.
So, it is unlikely that anything older is going to get 4k netflix support, firmware update or no. The device will lack the necessary hardware to do the decoding.
It all depends on the decoding chip in use in the TV. If it has h.265 support, it can be enabled through a firmware upgrade. If it does not, then not much is going to be able to be done to get it to decode h.265.