If you're anything like me, when you change from 4 thread to 3 thread you take the needle out and promptly lose it. This means every time you change from 3 thread to 4 thread you have to get out a new needle.
My Frister and Rossman machine has its own handy in built needle storage place, but most machines don't.
|In the front fold down section of my F&R|
If your machine does not come with its own anti-needle-losing device, try this simple trick. Cut a small approx 2-3" square of thick fabric like a fleece that will be difficult for your needle to slip out of. Put a tiny hole in the middle and slip it over your rightmost / lower looper spool.
When you take a needle out pop it into your needle holder and voila! When you go back to 4 thread you'll know exactly where to find your needle.
And the best thing about this, you can pop your spool on top and it won't affect the functionality of your machine at all.