Two lines, eight appearances, and they must not mean the same thing
The first and third lines of the opening tercet return eight times between them. If they mean the same thing on appearance six as on appearance one, the poem is dead by line 9. The refrains have to be re-read by their context: the same sentence, now said by someone who has learned something.
In the specimen, I keep the porch light on for no one now starts as a statement of habit. By the fifth appearance the dog has died and the line has become a description of grief mechanics. The words never changed; the reader did.
How to write a refrain that survives eight uses
Avoid a refrain that finishes its own thought. A closed sentence — the world is empty and I am done — has nowhere to go. Use a sentence with a hinge in it: a word such as now, still, always, or again, which can be re-dated by the lines that surround it.
Second, make sure the refrain can take both a literal and a figurative reading. A porch light can be a light. It can also be a habit, a vigil, a refusal. Eight appearances need at least three of those.
The rhyme is a constraint that produces the poem
Only two rhyme sounds across nineteen lines. In English that means the A rhymes will be drawn from a short list — now, how, allow, vow, plough, somehow — and the B rhymes from another — car, are, far, star, bar. The constraint forces syntax you would not have chosen: line 9 in the specimen, and still I leave the porch light on, somehow, is there partly because the previous line needed a B rhyme and somehow was available.
That is not a defect. The form is a machine for making the poem say things the poet would have avoided. When the constraint pushes you somewhere stupid, change the refrain in draft, not the rhyme.
The quatrain finally answers
The closing four lines are the only place the two refrains are allowed to sit next to each other with nothing between them. That adjacency is the form's resolution: the two positions the speaker has been holding for seventeen lines are, in the end, the same position.
If your quatrain needs to introduce a new idea to resolve the poem, the earlier tercets are not carrying enough and should be rewritten rather than patched at the end.