@IM0GEN, I was allowing for various costs - you can buy a house round here, in good condition, for about 260k that will yield you £900-950pcm - that's £9,000 over ten months, plus £1800 (the other two months) to cover the cost of insurance (about £200) repairs, a void period every now and again (during which, being retired, you save on outgoings by doing the bulk of any necessary work yourself - redecorate, fix the shed door, clear the garden), annual gas cert at £40, electrical check every however many years at about £200 including a few odd bits of work, the odd new carpet and so on. Every now and again you'll have to cough up for a new kitchen and bathroom and major repairs, but if you consistently salt a few grand away each year, you'll have it ready as needed.
£9000x4= 36000. So take off tax, and a few grand into savings for large expenses on the rentals. You've got about £28k a year, with no rent or mortgage to pay. That income will, barring economic or meltdown, broadly keep pace with inflation and your asset will also appreciate. It's not wealth, but it's totally manageable.
Let's also say that you take out a 40k mortgage across the 4 houses to cover stamp duty, fees and set-up costs. That will cost you a couple of grand a year, but if you put your mind to it, you can pay it off sharpish. Stay in work for an extra year or two and blitz it that way, if you want.
And if you have work pensions, when they kick in you can either enjoy the uplift in income, or sell a property and (after capital gains) have some ready cash.