The two major "Improvements" Perazzi did on type IV forend attachment are:
1) Lock the iron to the back of the wood to prevent movement during recoil to reduce iron/wood impact.
2) An interchangeable insert against the back of barrel under lug for easier barrel/receiver tightening.
These are the only advantages on type IV design.
Type IV only refer to forend attachment design, not the gun itself. Current top lever trip changes happened years later.
I had one type III gun that ran a 2" hairline crack in the forearm wood then stopped.
Another top single type III had an 30 year old crack ran the whole length of the forearm, and I shot it for another 10 years like that, nothing failed and nothing fell apart, I sold it as is, the next owner is still shooting it.
I Also have another 40 year old type III gun that went through more rounds than Lama Odom's bill at the Ranch (if we compare his bill vs today's AA shell's cost) that didn't crack the original forearm wood.
Some people like to make a big deal of it, but if you have an old type III forend that didn't crack the wood, just keep an eye on the retaining screw you'll be fine.
From what I've seen there are actually 3 forend iron types: the T1 with a tiny tab, usually broken off next to the knuckles, the T2/T3 with a modified face on the iron, and the T4. The T1 wood will not interchange with anything and the T2/T3 will interchange. The T3 can be identified by the center top position of the lever lock/release. I had a '72 T3 gun, but there may be some from 71. I don't know if there are any T1 guns after '68 but I've never seen a later one. JMO of course AFAIK the only internal change was the firing pins which seem to have gone thru a couple iterations but current will retrofit to 1968 with a minor nod to the top barrel hammer. See Whiz White's site for info on that mod. Another thing the meatheads should prolly avoid attempting. I've seen a couple that they obviously had a go at.
In addition there is the T4 action (?) if you want to call it that which began in the early 80's with the top lever release on the right side of the action. Just as are T1/T2 actions are distinct from the T3 by having the lever release on the left side of their action. I don't know what name there might be for the one piece action forging that started in the early 90's. Or if it needs one.
The T3 top levers are not prone to failure, they are prone to abuse by bozos who have no idea what they're doing and have no business touching a Perazzi. The T3 top lever is the same lock/release arrangement as the TM series was, is, and prolly always will be.