At 4 it's quite common for children to have lisps, often they will grow out of them, but at around 5 or 6 GP's would look at referring to a Speech Therapist for things like lisps.
Do other people understand your son?
Does he get frustrated that people aren't understanding him?
Waiting lists for Speech Therapy are generally quite long so if the lisp is severe and people really struggle to understand him I'd speak to your GP now.
If he's generally understood by other people most of the time and he just sounds young when he's speaking (which I suspect he might be) then I wouldn't worry too much and I'd wait until he starts school. Schools are good at picking up when a child needs help or if it's something they'll grow out of in time. At least our school is.
I had quite a bad lisp when I was 4, my family understood me but no one else did. I saw a Speech Therapist twice, she said I'd probably grow out of it when I started school and I did.
DD also had a lisp, I used to be a Speech Therapy Assistant, with adults though not kids, so no real expertise myself but I'd often chat to my boss about DD. Her lisp improved loads once she was in school full time, she almost 6 now, she still struggles with one sound but my boss and I are probably the only people who notice it, because we're listening for it.
The only sound I've actively worked on correcting with DD was a "th" sound which she said as "f". Her little brother's name ends in "th" so it was really obvious and irritating (to me). When learning phonics they learn that Naughty Clowns go "thhhh" they really emphasise it and stick their tongues out. We got her to really stress the "th" at the end of her brother's name and after about 2 weeks she was doing it in all words naturally.