Heads crack between valves seats because head was over heated, metal is thinner at those spots so when the metal expands as it gets hotter it will crack
Blown head gaskets are usually caused by overheating, leak in cooling system that is not noticed or a hose failing and sudden loss of coolant.
All metal expands as it is heated up, overheating causes extra expansion
The head metal gets the hottest, so it expands the most, and when it expands downward it Crushes the head gasket between head and block, and this can cause weakened cylinder ring in that gasket at that time or it can just "blow" at that time
When head gasket blows it usually causes an overheating cycle AGAIN because coolant is forced out by the "air" being pushed in, and the heads end up with just "air" and no coolant so get super HOT, same as running low on coolant
So heads may have already been cracked from first overheating but not as noticeable, or weakened from the overheating from what caused blown head gaskets to begin with
Blown heads gaskets are 99% from overheating, 1% from pinging/knocking, incorrect replacement, or bad gasket from maker
Cracked head is 100% from overheating
The 4.0l SOHC was used in several popular Ford models, and earlier years did have the failing timing chain tensioners so used replacement 4.0l SOHC often have several buyers, so wrecking yards do price them higher
4.0l SOHC didn't have the "cracked head" issues that the 4.0l OHV did, so yours is not a known issue like the 4.0l OHV, but can happen on any engine
On any engine, never try to "make it home" when temp gauge starts to climb, pull over, and wait for engine to cool down
Limp home in short trips or get towed, it will save you at least $1,000, and if it takes you 10 hours to get home you were making $100/hour, lol, not a bad payday