@WeightoftheWorld - I was just 31 when our twins arrived. DC 3&4. It hadn't crossed our minds although we joked about it the night before my 12 week scan.
As for practicalities. There is a nearly 4 year gap between oldest and twins. Ds1 was nearly 2, dd1 was nearly 4. Their birthdays fall in an 8 weeks period.
With 2 boys and 2 girls, room sharing has been easy in terms of arrangements but difficult in terms of actually sharing space. They now have bunk beds down the middle and a side each. We were also lucky enough to move from a 2 bed mid terrace where they all shared to a 3 bed detached with dining room and conservatory. This has meant they have a playroom they can share as well.
Car-wise, as they're getting older and bigger this has become more of a challenge. We are now looking for something more 'mini bus' like as fold down seats in the boot are ok, but it leaves limited leg room and therefore the shorter 2 are the ones stuck in back with the middle seats pulled forward.
Otherwise hand me downs and using baby goods from one baby to the next worked really well. It was knit because we had twins that we had to pay out a bit more. But friends and family shared and what went around came around. For years a friend of mine and I swapped clothes back and forth for the boys. She had them, gave to my eldest son. I gave back for her second and she returned them for twin son. They really tend to outgrow before ruining when they are so little. As they've got older, their clothes last longer and they all grow at different rates so it doesn't work anymore. But we had a good run for a good few years.
We made it work when our income was less. The kids were not aware of how tight things got and now they're older, I'm in a position where I was able to return to work and have a good job. Even though I was a sahm for a good few years. I have few regrets.