In V4, they've removed the light icon where you can turn it on or off since the tamagotchi can now turn off and on the light by itself. In its place is the new mail icon, where your tamagotchi receives different types of mail, ranging from fortune e-mails, gifts, and job alerts.
In my opinion, it's a good move by Bandai since the light icon does very little to the overall Tamagotchi experience as you usually only use it twice per day.
