The Perfect 4th Interval
The Perfect 4th Interval is a good next interval to learn because it is similar in some ways to the Perfect 5th, but different enough that you shouldn't have too much trouble telling them apart. Be sure to pick strong song references and to spend enough time practising before moving on - don't hurry - the journey is the destination!
To play a Perfect 4th, play a note on one of the thickest 3 strings, and then play a note on the next thinnest string in the same fret. See the diagram below for an example.
There are a few ways to play it and anytime you cross onto or over String 2 (B string) you need to jump the top note up a fret, it's one of the quirks about the guitar tuning. For the 4th it's only really when we go from String 3 to String 2.
There are quite a few possible song references you can use for the Perfect 4th, we're looking for the first two notes of the melody to use the interval jump of a Perfect 4th.
It seems that Auld Lang Syne seems an internationally recognised song but there are some further options below - use what you feel you know best - that's really important!
Dr Justin prescribes 5 minutes a day of singing a Perfect 4th (on top of your 5 mins of Play What you Hear). So 10 minutes a day of Ear Training if you can afford it!
1. Start with playing the interval on the guitar and singing along to get used to the sound and feel of the interval. Imagine your Song Reference while you are doing so, and sing the words if it helps. If you're still struggling with singing you can always hum!
2. Play and sing a root note, then stop playing and sing the interval jump and then check it on the guitar so you're sure you are getting it right and in tune.
I would recommend at least a week singing the one interval and using shapes all over the neck so that you get familiar with it everywhere - if there are parts of the neck you find a lot harder - work more on them. Play and sing this until you feel that you can sing it from anywhere and play the interval from any note! Then move onto the next interval!