How to find which users are currently logged in?

uptime

shows among others 4 users, should I be worried?

According to man uptime

uptime gives a one line display of the following information. The current time, how long the system has been running, how many users are currently logged on, and the system load averages for the past 1, 5, and 15 minutes.

Is there a way to find which four users are "currently logged on"?

The easiest method to find who is logged on to your system is the use the who command, a part of the gnu coreutils package. It can be used as an ordinary user with no options or with my own favored option which enhances readability:

andrew@ilium~$ who -H
NAME     LINE         TIME             COMMENT
andrew   tty1         2016-05-06 07:34
andrew@ilium~$ 

At times you may see yourself or other users apparently logged in more than once with the use of pts/0and pts/1 in place rather than tty1, This indicates a 'Pseudo Terminal Slave' (= pts) and simply shows that you or another user have opened an application such as xterm, sshd or similar process that creates a pseudoterminal master and slave pairing.

More information concerning pts in the referenced AskUbuntu thread, and the man page link from within the accepted answer:

• AskUbuntu: What does “pts/” in the output of w mean?
• pts(4) - Linux man page

All of these answers are excessively bloated. Three characters for a command? Please.

The fastest way to find who's logged on and doing what is:

$ w

You can use the command users to see who is currently logged in. Take care.