I know nothing about Cazalla de la Sierra, but it is a 2.5 hours drive between it and Almonaster La Real and 2 hours to Almonaster from Sevilla. Aracena is 45 minutes from Almonaster and Alájar is between them. The roads are rural and there should be minimal traffic. That being said you are doubling back if you head west only to turn around and head further east from Sevilla.
You need to consider time of year (what time is sunrise and sunset), driving time, and what you hope to visit in each place before deciding. At a minimum you have 4.5 hours of driving. I have not been to any of those towns/villages. Almonaster has its Mezquita and Aracena has a castle, caves, and ham museum. Alàjar is a wild card as it doesn’t have any sites that shout, visit me. That area is known for Jamón Ibérico.
Being one who frequently stops and visits small villages and hamlets, many can be seen in an hour if all you’re doing is walking around. Add something of interest or stopping for a bite to eat, an hour can easily turn into two or more.
To save some time, look up free parking lots before the trip and bookmark them to your navigation system so you can go right to them. All small towns seem to have free parking lots but you need to know where they are since they are generally not well marked.
To your original question, that’s more driving time than I like in a day, but for a one off day trip it is very doable and I say go for it.
Remember that the driver needs an International Driving Permit. Finally, adhere to the speed limits. Going only a few kilometers over the limits can get you a ticket and cameras are all over, especially when entering and leaving small towns.
Thanks to your post I added the castle at Aracena to my list of places to visit. The Mezquita was already on it.