Dental Implants 4 Years After Tooth Removal?

No, Too much time has not gone by.  The only issue is that you may have some bone loss where the teeth were removed.  This may necessitate some bone grafting procedures either at the time of implant placement or prior to implant placement.  This may include sinus lift bone grafts for the upper teeth as well. There needs to be adequate bone width as well as height below the sinus floor on the top jaw and above the nerve on the lower jaw. The recommended plan would be to have a Cone Beam CT Scan done which is a 3D radiograph (low radiation) to determine exactly what needs to be done

The amount of time that has passed does not matter for implant placement.  What is critical is if there is adequate bone present in the sites to support an implant. I would suggest a 3D cone beam image of the areas to best evaluate the amount of bone present, position of the main nerve in the lower jaw, and the position of the sinus in the upper jaw.  If inadequate bone is present grafting may be an option.

There is no reason why implants cannot be done.  If there is not enough bone, a bone graft can be done to support the implants