Actually, your fretting hand has 4 fingers and a thumb!

What's so hard about using the pinkie?

Beginning guitar students are way too eager to avoid using the pinkie finger at all costs. It won't stretch that far, it's not strong, I can't get it to work right, and who knows what else? I've watched ______ play and he or she doesn't use it.

Practice stretching exercises and quit avoiding it.

When you're serious about learning and improving your guitar playing, you use all of the tools available to you. Your guitar teacher can give you plenty of stretching and strengthening exercises to get the pinkie moving. Start out on frets 9 through 12, playing them in both directions: 9-10-11-12 and then 12-11-10-9, and be sure to use all 4 fingers. The other secret is to leave the 1st finger down through the whole sequence. When you can do that comfortably on those frets and all 6 strings, move to frets 8 - 11. You'll be pleasantly surprised at your progress.

Should I use my thumb?

While classical purists frown on thumb use, many guitar players use the thumb to fret the 6th string. You'll be in good company joining Jimi Hendrix, Chet Atkins, Mel Bay, and many others in using the thumb. I use it on some difficult chords as well as in songs when it's convenient. Angie, by Bert Jansch and recorded by Simon and Garfunkel is an example of using the thumb. I use the thumb to play the barre chord F at the first fret. Guitar students with smaller hands may find it more difficult, but the best thing to do is try it and see what happens.