Use a cotton swab dipped in rubbing alcohol to clean all parts of the atomizer, especially the threaded end that makes contact with the battery. Now take a tissue and hold it to the threaded end while blowing into it. You should see some tan colored liquid on the tissue. This is the e-liquid or nicotine residue that is not vaporized. You may get some e-liquid on your lips or in your mouth and it will taste bad but it will not hurt you. (You actually should always blow this out after refilling. Because of course, you have removed the device from the battery before filling!) Don't use regular tap water to rinse your atomizing device with as it contains far too many items you do not wish to collect inside. Use distilled water to soak your device, and rinse anything that you can get at easily without flexing anything or moving anything around in a manner that is not natural for the device. Always clean the mouthpiece separately. Items that have removable cores are much easier to clean as you can actually get the water to flow more easily into areas. Around the house here we've found that items with wicks function in a superior manner if the wicks are rinsed about every other day. The wicks are very delicate! We learned they can tear easily. If you decide to take your items apart to clean them, you must accept the fact that they might never go back together again properly! So take your time and work over an area that if something is dropped, it does not go down a drain or onto a floor with dog hair.
Some people choose to spend their time vaping and not cleaning. That is fine, too. Just be prepared you will have to replace things more often. Either way, even if you used a new ce4 every day, chances are you are still saving money, and of course your health. If you ever find yourself in a situation where you think your atomizing unit has declined, and it's full of liquid, you can fairly easily remove the liquid. If you take your bottle with the blunt nosed syringe end, or a syringe with a dispensing tip, you can suck the liquid out. For the bottle, press it in gently, hold it, bring the tip of the needle into the liquid, and slowly release the pressure. This should get most of the liquid out so you can begin to investigate what the issue is, or toss it in the trash!