I taught art for several years and I think 3 or 4 hours for 1 project would be a lot for even my high students to focus on.
The 3rd and 4th graders loved working with textiles. Weaving seemed to be the favorite activity. We did bags, belts, bracelets, key chains& After the students learned the basic idea behind it they got to choose the project they worked on. A teacher I worked with also brought in her sewing machine and would help the student make a pillow from their weaving. The students had to bring in an old pair of jeans they could cut up. The pillows turned out great, just make sure you student makes a large weaving.
We also used burlap to create ship pictures. The kids had access to felt, buttons, yarns and all kinds of items they could sew to the project. Of course, it does not have to be limited to ships. I could see this project working well for any subject. They love choices, it makes it project their own.
Clay is always a hit at every age. One of the favorite projects was making a bell. Make the bell portion from a pinch pot, allow the kids to sculpt the handle (I always narrowed it to animals) and have the early finishers make beads for everyone. The kids always loved these, I felt like I was able to teach a bit of basic hand building and they got to make a little sculpture which is what they want 98% of the time.
Edit to ask... Is it 3 or 4 hours in one sitting or 1 hour a day for 4 days? 3 or 4 hours in one block is too much but 1 hour over 3 or 4 days is totally doable and will keep their interest. Just have an alternative activity for your earlier finishers.