> Does it reflect badly to leave my role before a year is out?
No. I think 4 months is long enough to know whether a position is a poor fit. Resign with this reason, work your notice and move on.
The problem here I think isn't the fact that you are leaving within 12 months, this happens all the time, the problem is that your next position is likely to be the same.
There's no perfect software job. Poor practices and technical debt is everywhere. Even these big name startups will have to grapple with these problems. Most people are not motivated enough to go above and beyond to fix these problems, because it's not their company. Why would I reduce the test suite runtime to below the current 90 minutes? Why would I refactor or pay off technical debt? After all the features have been shipped. The boss is happy.
I've often felt as you do, wanting to start my own business to implement what I see is the best practices I've come across. The only thing stopping me is a lack of a good idea and that I need to make rent and take care of my wife which is making me very risk adverse.