The best way to change this to to create a routine for him, which means you need to structure your schedule to take him to where you want him to eliminate often & at the same time every day/night.
Since he's 4 years old, but not used to this, I'd suggest these intervals start with every 4 hours. If you want to start more frequently or he is having accidents, then maybe it needs to be like every 2 hours. Gradually the time in between bathroom periods can be lengthened to 5,6, 7, 8 hours. Personally, I think it's in the best interests of canine health to eliminate every few hours every day and only hold it overnight for longer periods (7,8,or 9 hours). The place that you designate and lead your dog to every time for elimination does not have to be outside. It can easily be a disposable potty sheet or a reusable pad. There’s all different types including some that are washable and some grass alternatives. Give lots of verbal praise and happy petting for eliminating well.
In between those elimination times, you need to restrict his movement temporarily, until he learns with this new schedule. You can crate him with comfy bedding (to make crate like his den). Crate size needs to allow him to stand up & turn around in & lay down, but not so large that he could eliminate in one spot inside and still relax in a dry spot elsewhere inside. Another alternative some people use is indoor fencing /playpen/ gates on an easily cleaned floor, sometimes with pads underneath. That option can be trickier because sometimes the dogs will chew the pads and if the fenced/gated area is too large they can still eliminate in one area and relax in another. The other option, which probably takes the most of your focus, and maybe would be easier to implement after you’ve started this process: is to use his leash as a tether to you- wherever you go, your dog goes. The same principle applies: you take the dog to elimination spot on schedule/same time daily every few hours, but if you see sign that your dog needs to relieve itself (because one end of the leash is attached to your arm, wrist, waist or leg and the other to the dog) you immediately take to eliminate.
Generally, dogs don't want to be in their own urine& feces, so unless there's medical reasons they will not eliminate in their sleeping area.
Addition: with the fitted sheet you ordered, my guess is it can be laundered. Adding the pup's bed is recommended, so it can sleep and be comfortable. I'd suggest not adding food. Unless there are too many hours in between, I wouldn't include water. The food you want to serve and have the dog eat at approx. same time every day. Following eating and drinking water, you'd want to encourage the dog too eliminate. However, if the dog is in the ... (more)