I have no idea how the Sims 4 would implement physical disabilities with how some of the worlds work - Discover University introduced bikes, which is really strange considering the entire world outside of builds is not wheel friendly. For all we know, they might be working on such things, but given the limitations I've seen with just building in build mode... the game would need a whole lot of new features. and possibly a reworking of all the worlds.
A worrisome things is that the Sims 4 is really limited on their animations. They do not put in the sort of effort you see in previous releases. They reuse and rehash ones from a decade ago to this day. For physical disabilities they will have to step up their game to make new ways for Sims to function.
They would need to create whole new walking styles for Sims who might use canes, walkers, crutches that take things like going up or down hills, stairs and the like into consideration. For wheelchair bound Sims, there would need to be new animations in regards to accessing things from certain heights, being able to transfer to seating, beds, toilets. Many builds would likely need to be modified to have entrances wide enough for wheelchairs and ramps that adjust to seamlessly with any porch or platform height. Foot prints of most things also would likely need to be adjusted to ensure they don't get stuck - a lot of builders sure do love to cram as much as they can into one room and follow the "you only need a half tile of space to get them through." Clothing would probably need adjusting as well, especially long, puffy skirts when it comes to wheelchairs. Well, unless players are fine with a lot of clipping happening. Afterall, even now half of Sims' bodies will pass through walls, and depending on your Sim's body type, their backs will clip through chairs.
Thinking on it more, depending on how different the physical disability is in terms of mobility, there would likely need to be a special tagging system in place as well to identify and ensure the Sim doesn't do certain behaviors by accident - like suddenly jump to the floor to do sit ups or pushups, break out in a run when scared, suddenly jump into a the arms of the Sim who proposed to them and the like. There would also need to be new animations added to fully able-bodied Sims for proper interactions to take place such as displays of affection.
This sort of expansion regarding such disabilities will likely take the team an exceptionally long time to produce if they want to do it right. In turn... I question just how long it would take for them to introduce physical disability to the game when so many Simmers are displeased with how many things that were present in the older Sims games have yet to show up after the passage of a decade.
