I have a 4 year old cocker spaniel that has never been socialized. I am trying to get her interested in toys but she won't play....?

Being uninterested in `toys` is nothing to do with a dog not being socialised.
Some dogs just don`t have any interest in playing with toys.
It can often be put down to a low prey drive, although not always.
At 4 years old your dog is not going to suddenly see the point in playing with toys, and if she does do it for a short while it`s usually with half a heart.
Be a bit thankful as sometimes owning a dog that is `toy mad` can be quite annoying when all they do is keep plonking toys in the owners lap when the owner is trying to watch the TV. 🤣
Believe me....I know.

I rescued a dog at 8 weeks old. She has NEVER picked up or played with a toy, never. She isn't the least bit interested. If I throw a ball she watches it and then looks at me as if to say, "What was the purpose of that?" She DOES play, run and wrestle with my other dog.

She will if you act like you want the toy.

Do you ever bring her to a dog park.

If she is a rescue, she may never have learned to play. It will take time & patience. Some dogs have less play drive than others, however. So I (or we) have no idea - which boat you are in.

You indicate "when she does play" it just doesn't last very long...but you do not bother to tell us what has worked, and what has not. Do you praise her, laugh, sit ON THE FLOOR .....WITH HER?

A lot of MY dogs like soft squeaky toys. The problem is, they don't last long if I do not pick them up - as they get "gutted", so we have "play sessions". Kong still makes some soft toys like this.

With puppies, I usually try games of "KEEP AWAY" with a towel being dragged on the floor - via a piece or string. I can make it jump or wiggle, etc. (I don't know if she would like that or not, or like it better if this was a squeaky toy like a long skinny fox or worm.) The Audubon company made some life-size or larger stuffed toy BIRDS that make bird sounds (via a battery) - and for sporting dogs; these can be fun.

You might also want to try scent games or nose work - where you hide some treats & encourage her to FIND them. The games (hiding places) can get harder, as she gets better at finding the treats.

Not all dogs have a strong retrieve drive. While it is great if they do, you may find she likes a suede or leather glove BETTER than a tennis ball - when tossed. (I do tracking, and we are ALWAYS finding uses for orphan gloves..... we don't go out and ruin a perfectly GOOD pair!)

Finally, playing WITH another dog - (who does know how to play) can help her. She will watch its interaction esp as it works for its owner and may see the dog having fun - and want to do something it sees.

Socialization has nothing to do with playing with toys. Some dogs just don't play. You may have unrealistic expectation of your dog. Try getting a few of the interactive dog toys/puzzles to keep her busy & learning.

You can start socializing her now. Any good reasons as to why she was never socialized.

My dogs rarely play with toys. It has nothing to do with being socialized. They're just never bored.

Some dogs do some dogs don't. It has absolutely nothing to do with being socialised.

You can never know for sure. Just try her on some ball treat despencer or tennis ball. I don't know for sure with my dog. If she used too play I would get her to the vets. But she just might not be the type to play

just reward her and give her treats when she does play

A lot of pure bred Cocker spaniels are outright irritable, some simply just don't feel good.