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[PlasmaKrab]

You're right, there must have been either high attrition during the war or a typo in one of my sources. 1000 Chieftains would be the bulk of the tank force to replace after the inception of the Shir. Upgrading them to Mk11/12 standard or above would be interesting.
Of course, this considered, the whole Patton fleet would be a secondary concern.

What I meant with a 4th generation tank was a brand new post-2000s one. Only working example I can think of is the Korean K2. The Black Eagle is another project but not yet in its final stage. Make the base requirement 120L55 gun or better, high mobility, hunter-killer/autotracker FCS suite with 2nd-generation TI on both sights, active protection... on a newly designed basis. Don't ask for more details, I'm not too good at this generations game. The idea is just to get a newer design than upgraded 70s/80s tanks (M1A2, Leo2A6, Challenger2E...) which would be not much better than whichever upgrade you could lay over the Shir 2.

[Marcello]

"You're right, there must have been either high attrition during the war"

Attrition of Chieftain was very high indeed, the iraqis ended up with whole lots filled with them.

"Make the base requirement 120L55 gun or better, high mobility, hunter-killer/autotracker FCS suite with 2nd-generation TI on both sights, active protection... on a newly designed basis"

Frankly nothing of the above requires a new chassis. The gun can be fit in a Leopard 2 or Chally 2 with no problem. The rest is electronic stuff you could fit on pretty much anything.

[Marcello]

A new chassis is really needed only if you want radically better protection and a design optimized to take a 140mm from the start.

EDIT
Well you could stick a 140mm in a Leo if you wanted (it was done) but I suppose it might be better to have an optimized design with an autoloader from the start. Uparmoring the tank to survive 140/152mm hits is an other matter. Therefore you will probably need a new chassis with some exotic configuration, otherwise there is no need. The K2 is basically an equivalent of the
Leo 2/Chally 2 with a few funny features.