Curiously, I changed the last leg to early morning on 5th September (so less than 24 hours after flight 3) and the price stayed the same.
As Smid says, that would normally increase the price but that isn't always the case - it's down to how the particular airline has priced the fare classes on that particular day or days. If you're on a BA final leg in Club Europe/Club World, for example, then if it's within 24 hours of the previous leg then it will quite likely be a different booking code than one where it's more than 24 hours ... the prices of those booking codes are usually different on the same day but not always and, of course, a "within 24 hours" on one day is not always more expensive than a "more than 24 hours" the following day (or a month or two later, which I once booked if you go all the way back to the top of this thread!).
I made some ex-DUB bookings last year with VS UK-US wrapped in BA DUB-UK fares where it worked out cheaper to do two different combined DUB-UK-US-UK-DUB business class bookings but pay the bit extra to 'stretch' the first combo UK-DUB final leg by a few
months to be the 'positioning' flight for the following DUB-UK-US-UK-DUB meaning it saved me the cost of the UK-DUB 'positioning' flight for my second trip, gave me that flight in Business instead of doing an Easyjet cheapo and also meant my first trip was fully completed (so no risk of losing miles / TPs retrospectively) as well as being no discussion whatsoever on retrieving my luggage at DUB-UK-US-UK point on my first combo. Booked those two different combined DUB-UK-US-UK-DUB business class bookings on Expedia Ireland but other sites are available.