T.H.
Honestly, at 10 months, they think everything is cool, so I'd cater a bit more to the 4 year old. This past summer we went to northern Wisconsin and rented a cabin and did fun beach/swimming things. You don't necessarily have to go that far, but find someplace beachy that's near you. My son was 10 months at the time and loved it! He had a lot of fun crawling through the water and banging a shovel and pail around. He did try and eat the sand, but hey, ya take the good with the bad! ;) You could also do a hotel with a water park or someplace that has a great zoo, parks, etc. I don't know if you're wanting to travel or stay near home, but I think anything with water, whether indoors or out is great for all parties involved. Both the kids will probably love it and it will wear them out so the adults get some quiet time too! Have fun!