Signs that your four-year-old may be delayed may include if you're unable to understand they're speech.
At this age they should be understandable so you should be able to understand what they're saying and if you can't, that's cause for concern. If they're clumsy in the playground, if they're falling a lot, if they're unable to keep up with their peers in the playground, that's cause for concern. If they're unable to hold a crayon or resisting to holding a crayon or a pencil that's cause for concern. If they're resisting to be dressed or getting dressed themselves, using the toilet by themselves or sleeping, that's cause for concern.
Another sign that your four-year-old may be delayed would be if they're not engaged with their peers. This is the time where they should be playing and wanting to play with other children. If they seem withdrawn, if they seem that they are not engaged or not willing to engage, if they're running off into the corner, if they're resisting to even being held, that's also a cause for concern.
And remember, you also have something called your instincts and if something doesn't seem right there's no reason to delay speaking with your child's pediatrician and discussing your concerns with them.