Welcome! 
sounds like an exciting project, and I’m sure you’ll get a lot of good advice.
I’m not feeling very well today and since I rely on text to speech, I missed some of the details in your post, so I’m not going to comment on the specific layout at this time. Again, I’m sure you’ll get lots of other answers.
I did just want to address three quick points. First, if you are using SmartThings, you can definitely have voice control of everything that smartthings controls, and that includes the fan speed. We do need to know what country you are in as device options do vary somewhat. 
Second, if I understood your question correctly, you wanted to know if four-way set up is possible with home automation. The answer is definitely yes, it all depends on exact devices chosen. For example, you might even have one of the switches as a battery operated switch which looks just like the other and can be stuck anywhere. But more commonly there’s a way to replace the existing wired set up. 
Third, if we can just take a step back for a minute, there are three common ways of making a mains powered device smart.
A) The Lighting device itself has a radio inside it and can accept commands from the network. This is most commonly a smart bulb. These come in many different shapes and sizes, including recessed can lights.
https://www.amazon.com/SYLVANIA-Recessed-Lighting-SmartThings-Assistant/dp/B0196M601A
B) there is a small device called a “micro” which is placed inside the wall on the specific circuit branch controlling one device. For example, there might be one inside the ceiling light fixture or Inside a fan canopy or behind a wall switch. This can be especially useful when you want to have individual control of multiple devices on the same circuit.
C) You can get an all in one smart switch that replaces an existing wall switch. There are lots of these in many different styles. The ones that can control a fan have a different internal mechanism than the ones that can control lights but they could look the same on the outside If you like that aesthetically. Or the fan controls can look quite different. But there are many options. If you use SmartThings, the switch will probably use Zwave or ZIgbee not Wi-Fi. ( that’s a good thing, for several reasons.)
https://www.amazon.com/Repeater-Enabled-Samsung-SmartThings-Inovelli/dp/B01NAUCQ18
There’s lots of information available on different brands and different options and all that, we can discuss that later.
Bulbs, switches and sensors, oh my....what to buy (device class features FAQ)
But I just wanted to start out by mentioning these three different approaches. Many people have all three kinds of devices in their home, with different ones being more suitable to specific locations/uses. And being open to all three give you a lot more flexibility in how you lay things out.
So you may get suggestions which include any of these three kinds of devices, and I just wanted to make sure it was clear what people were talking about before we got into the details of your own project. 
Tagging @jody.albritton