#4 seems very common for my parents' generation (in their 60s). #1 seems like a logistical headache for something like a mortgage.
My ex-wife and I basically followed #2. This works fine for a lot of people. It worked fine for us until it didn't - unrelated to the OP, but if your spouse runs up a shitload of CC debt while you're married, you're responsible for half of it. If she cashes in a 401(k) and uses the entire proceeds to pay off said debt, you're responsible for half the taxes.
My current SO and I have discussed merging accounts will probably do something closer to #3 - each of us putting in somewhere between 50-75% of our income into a joint account, and the rest is personal money. But the joint account is for everything. Obviously joint stuff yes but basically anything that isn't 100% individual will come out of that account.
Edit: No kids though.