If the neuter was done properly, then no it's not remotely possible - and in that respect if it was done 4 years ago but not properly, chances are she would have had other kittens before now if she's been outside at all.
I guess it's still worth following up though "just in case" - how old are the two kittens with her, are they definitely hers? If you don't follow up you'lll probably always be wondering whether it was her, so even though I wouldn't get your hopes up, I would say check it out.
I do hope you manage to find your loved one xxx