JavaScript kullanarak yapabilirsiniz.
let domain = "www.r10";
let randomChars = "";
const possible = "abcdefghijklmnopqrstuvwxyz";

for (let i = 0; i < 4; i++) {
randomChars += possible.charAt(Math.floor(Math.random() * possible.length));
}

let fullDomain = domain + randomChars + ".net";
console.log(fullDomain);