I bought my 4th gen TV last January. The new TV 4K has a faster, more powerful processor, HDR for better colors and 4K for sharper movies. Aside from that they’re pretty much identical.
For me personally, the new TV is not enough to upgrade to. For others, HDR and 4K alone is enough reason to upgrade.
It comes down to financial situation and personal preference.
Go for it if you can afford it and really want HDR and 4K.