First you have to take a look at the note G on the fourth string and where you can find it on the second string:
1. ----------
2. --3-------
3. ----------
4. --0-------
This means that when you use the G on the second string you can replace it with the G on the fourth string. We will start with playing a little C-major scale the ordinary way.
1. -----------------0--2--3--
2. --------0--1--3-----------
3. --0--2--------------------
4. --------------------------
Now we can replace the second string G with the fourth string G:
1. -----------------0--2--3--
2. --------0--1--------------
3. --0--2--------------------
4. --------------0-----------
If you use your right hand thumb to play the fourth string and the index, middle finger and ring finger to play the other strings you can create ringing and interesting patterns by using the fourth string G. Here you have an example:
The symbol h means hammer-on and p pull-off.
1. --------------0------0---------------
2. -----------1------------1-------------
3. --0--h--2-----------------2--p--0--
4. ------------------0------------------
Well, now it's time for you to create your own licks!
Wikipedia information:
Hammer-on is a stringed instrument playing technique performed (especially on guitar) by sharply bringing a fretting-hand finger down on the fingerboard behind a fret, causing a note to sound. This technique is the opposite of the pull-off. Passages in which a large proportion of the notes are performed as hammer-ons and pull-offs instead of being plucked or picked in the normal fashion are known in classical guitar terminology as legato phrases. The sound is more smooth and connected than in a normally picked phrase. The technique also facilitates very fast playing because the picking hand does not have to move at such a high rate, and coordination between the hands only has to be achieved at certain points. Multiple hammer-ons and pull-offs together are sometimes also referred to colloquially as "rolls," a reference to the fluid sound of the technique. A rapid series of hammer-ons and pull-offs between a single pair of notes is called a trill.
