> It had all the room it planned for and needed at the start.

Almost. Come S4 they got concerned the show would be canceled before finishing the story, so they dropped all the secondary plots from S4 and S5 and compressed the main plots from S4 and S5 into just S4. Then they got renewed and quickly wrote a new S4 finale using S5 budget, postponed the series finale a year to S5, and reworked all the dropped secondary plots into S5.

Personally I think this worked really well, made S4 much more fast-paced and S5 feels more like a "the world keeps turning" extended epilogue.