I think your question about the 4K to 5K hull, would best be answered by
@Anmdt.
The in-between tonnage platform has been on the board for quite sometime, albeit having been put on the back burner a number of times.
It is however, the logical way to go.
Considering the proliferation of the anti ship missiles of all types, be it subsonic or supersonic or even hypersonic; They are going to be the main adversaries of any large seafaring assets you will have in your navy. Oceans and the seas of the world were big and endless expanses in the 1940s. You had to send planes searching for enemy fleets. Nowadays, satellites and long range radars on planes have rendered the seas and oceans all accessible. Your billion + dollar assets will be the number one targets for those missiles. They will be the first casualties of any
modern war. I am not even going to mention the deadly submarines with their AShMs and modern long range torpedoes.
So, how clever is it to invest in these large and expensive ships? There is no place to hide in the sea, unless you are deeply submerged. If we had to invest in surface combatants, I would rather invest in two ocean faring smaller, 4 to 5K ton frigates than one large destroyer. Our last decision for a further 4 I-Class ships making the total of 8, was the right move.