Since it is way easier for a 4-level laser to get population inversion than 3-level laser, 3-level laser is still in use nowadays. So what's the advantage of 3-level laser over 4-level laser?
I guess you should take the view of an experimental physicist:
- Which atoms are available and where are their energy levels?
- How can I excite the atoms in order to get inversion?
- What are the properties which I need my laser to fulfill (e.g. line width, power, ...)?
- How complicated and stable is the set-up? How much does it cost and how long will it take to get the laser running?