Looks like account.arena.net 4 digit code possible combinations are numbers from 1023 up to 9876 which do not have reapeating characters

arena.guildwars.net.py

import http.client
import json
import datetime
from multiprocessing.dummy import Pool as ThreadPool


def performGetRequest(path):
    connection = http.client.HTTPSConnection("account.arena.net")
    headers = {
        'Cache-Control': 'no-cache',
        'Referer': 'https://account.arena.net/register',
        'User-agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.99 Safari/537.36'
    }
    connection.request('GET', path, None, headers)
    response = connection.getresponse()
    r = response.read().decode('utf8', errors='ignore')
    return r

emailPrefix = 'sokhach'
emailSuffix = '%40gmail.com'
name = 'sokhach'

pool = ThreadPool(100)
indexes = range(10000, 30100)
found = False
count = 0

def process(index):
    global found
    global count
    #if found: return
    count += 1
    jsDate = int((datetime.datetime.utcnow() - datetime.datetime(1970,1,1,0,0,0)).total_seconds()*1000)
    response = performGetRequest("/ws/username?_t=%d&email=%s%s%s&displayname=%s" % (jsDate, emailPrefix, index, emailSuffix, name))
    parsed = json.loads(response)
    code = parsed['suffix']
    #print("%s - %s (%s)\n" % (code, index, count))
    #print("%s\n" % (code))
    # if code == "7777":
    #     found = True
    return code

def f2(seq): 
   # order preserving
   checked = []
   for e in seq:
       if e not in checked:
           checked.append(e)
   return checked

fullRes = pool.map(process, indexes)
distinctRes = f2(fullRes)
distinctRes.sort()
print(distinctRes)