Q: If I leave my shredded leaves in a plastic bag outdoors and punch small holes in the bag so rain water will wet them, will they compost by spring?
A: The leaves will eventually decompose into compost but I think it will take longer than four months. Most of the bacteria and fungi that break down leaves require warm temperatures. They work very slowly in winter. In addition, the best decomposing organisms require lots of oxygen, which would be limited in the bags.
Aesthetically and practically I think you’d do better by piling the leaves, sans bag, in an unobtrusive corner of your yard to compost as nature intended. If you want to speed up the process, make sure the pile is at least four feet tall by six feet wide. Dampen the leaves as you construct it and sprinkle a pint of 10-10-10 on top when you’re finished. “Black gold” will result by late spring.