These two sentences are ok, but as friendly advice to hotel customers, you should make them sound more formal. Here I offer my version for your reference.

1. Please close the door all the time for the security.

Please always remember to lock your door for better security.

If you forgot  (no need to use the past tense) the passcode of the door lock, please come to the front desk on the 4th floor.

If you happen to forget the pass code to your room, please ask the receptionist on the 4 th floor for help.

2. If you want to open the door, push the small button with a heart sticker on it.

The door can be opened by pressing the small button with a heart sticker on it.

Hope this helps :)