Dini,
Basically, you can do ALMOST what you said.
Diluting it 3:1 with VG will give you 25mg e-liquid, at 75%VG/25%PG. (Not 4:1 as you stated, which would give you 20mg).
However, you probably don't want to be mixing up 4-liter batches. Shake the 100mg bottle very well, then pull out maybe 50-100ml of the nic base, add 3 times as much VG, and then shake or mix very well. Keep the 100mg stuff closed up (I.e. put the cap back on as soon as you've finished pouring). Mix in a place where spills won't be a problem (I use a shallow metal baking sheet). Mix when you can concentrate, and you won't be interrupted, so you don't get confused about what you've added. Label the result with the concentration.
There are differences of opinion on how dangerous the 100mg e-liquid is; I started vaping on 36mg liquid, which is 1/3 that concentration. But for some, that would be way too high. And if I get a little bit on my hands (I haven't yet) I'd just wash it off. (I certainly have gotten some of the 36 mg on my hands!) I think it's worse to panic, in a case like that, where you might instead knock something over and make things worse. (And if gloves affect your ability to screw on caps, etc, they may do more harm than good).
Make sure the nic liquid is stored safely away from children and pets, and if you're storing it in the freezer, make sure it can't contaminate any food. But these are rules you should follow for any e-liquid, in any case.
Be organized, and mix liquids in an area where you have plenty of uncluttered space, and good lighting. Put stuff away as soon as you're done with it.
Do read up on DIY here, there's lots of info available.
And Welcome to ECF!
