Four houses, none of them near each other, and every one of them better than the museum it should have become.
Book the ticketed ones first and let the day arrange itself around them.
These four are not a day. Leighton and Pitzhanger are both west and can be paired; Soane and Dennis Severs' are both central-to-east and can be paired. Crossing between the two pairs eats an hour and most of your patience, so give it two mornings rather than one long march.
Soane by candlelight is the single best hour in this directory, and the reason I keep coming back. It is limited and it is booked, so treat the ticket release as the appointment and the evening as the consequence.
Dennis Severs' asks for silence and means it. Do not take it as a novelty and do not take a companion who talks; the whole conceit collapses the moment somebody explains it to you in the room.