Most common 4 letters in WCA ID

import csv

def common_id():

    limit = 100

    with open('WCA_export_Persons.tsv','rb') as tsvin:
        tsvin = csv.reader(tsvin, delimiter='\t')

        id_list = []
        count = []

        for line in tsvin:
            wca_id = line[0][4:8]

            if wca_id not in id_list:
                id_list.append(wca_id)
                count.append(0)

            i = id_list.index(wca_id)
            count[i] += 1

        c = 0
        previous = 0
        for x, y in sorted(zip(count, id_list))[::-1]:
            c += 1
            if c>limit and previous != x: break
            print str(x).zfill(3), y
            previous = x

common_id()