word riddle

This was in SciAm:Mind, which 'Brozy grabbed for me the other day.

In the word gum, every letter can be replaced by y to form a valid word, as follows:

gum
yum
gym
guy

There are about forty such three-letter words, but a script I threw together only finds seven such four-letter words: two with e, two with l, one with o, and two with r. Can you find them?

My solution script follows. Remove the pound signs for the answers. (Download the /usr/share/dict/words I used.)

#! /usr/bin/env python

testlen = 4
dict = {}
f = open('/usr/share/dict/words', 'r')
for line in f:
        line = line.strip().lower()
        if len(line) == testlen:
                dict[line] = 1

chars = {}
for i in range(97, 123):
        chars[chr(i)] = 0

for word, v in dict.iteritems():
        for c, found in chars.iteritems():
                found = 1
                for j in range(testlen):
                        mod = word[:j]+c+word[j+1:]
                        if mod not in dict:
                                found = 0
                                break
                chars[c] += found
                #if found:
                #       print word+': '+c

print chars

Addendum: the o solution and one of the r solutions require proper nouns. Also, the o already has an o.