I totally get wanting them to go outside! They are SO gross.
I don't think they are ready for 40-degree temps. HOWEVER, you can still get them outside if you have a protected area for them. Put a heat lamp in a doghouse or similar and stick it in their enclosure. I have done this with 4-week-old babies in cool temps, and they do fine. Your biggest worry is them getting wet AND cold at night, so make sure the doghouse is large enough for all of them to get in together. Also, check on them regularly the first night to make sure they're sharing--sometimes a few ducks will hog the doghouse and the others will get chilly outside.
Do be careful to protect your electric cords from the weather (rain especially), and not to let the heat be anywhere near the straw (put only a thin layer of straw in the doghouse, and hang the lamp securely from the highest point possible, leaving as much headroom as possible.
This has worked for me with multiple batches of older babies. With younger babies, I send them outside to wire-bottomed brooders as early as a couple of days. I do line the bottom with insulating material (a thick layer of straw, sometimes on top of or under a warm towel) at night (leaving the area around the waterer open-bottomed so the bedding doesn't get wet), and put the heat source in a protected corner so they can get good and warm during the night. I do NOT do this if it's going to be freezing weather, but 40s seems to work fine. I do make sure the heat lamp is hot enough--a 40 watt may not be enough. I also usually have TWO lamps out there because if one burns out I could easily wake up to dead ducklings. I had a batch of quail almost die one night because although the lamp was on, the bulb was a 40-watt and not low enough (I thought it was 100, but didn't actually check before leaving them for the night). In the morning, they were in a pile up against the wire that contained the heat lamp, and all of them fluffed and miserable with cold. I felt SO bad!
Anyway, experiment with it. You may be up in the middle of the night a few times the first couple nights, but it's worth it if you can get the stink bombs out of your house!