I want to drain an aluminum downspout into a 4" PVC pipe. How can I seal the space between the downspout and the pipe so that leaves and dirt do not fall into it?
There are plastic fittings with a rectangular opening on the downspout end and are round on the other end.
Jim Stewart's answer covers the adapter, but let me suggest some bigger picture considerations. If you're going into solid pipe, debris tends to wash through, and they're easy to snake out if debris collects inside. Corrugated pipe tends to trap debris and form huge clogs that are difficult to get out. I've had to abandon in place buried corrugated pipe that got clogged. If corrugated pipe isn't buried, it's pretty ugly. If you are going to bury the pipe and do a long run to somewhere far from your house, use solid pipe.
If you're just trying to get water a reasonable distance away from the foundation, there's a less visible alternative that doesn't require trenching: flattened extensions that hug the ground. I switched to these after buried corrugated pipe had to be abandoned. Home Depot carries ones that look like this:
A flexible adapter slides over the downspout. It can be stretched and angled as needed, and connects to flat extensions. You can add as many extensions as you need (sold separately).
You can hide the extension under mulch, stones, or other landscaping cover. However, the flat, corrugated shape does tend to catch debris, so I leave them accessible. To clean them, you snap them apart and shake the debris out. They're more expensive than either form of round pipe, but they work without being very visible.
My house has mulch landscaping beds around the foundation. I use enough extension sections to get across the bed, or about 8' from the foundation. This is far enough, and isn't a mowing problem.