How safe is the android Lock Screen with password? If your tablet got stolen, how probable is it, that the thief will retrieve data from your device if you have only the lock-screen with a 4-character password as protection?

Is it easy for a hacker to get past that? Or can I lean back and be sure, that stored passwords in Firefox etc. cannot be retrieved?
I wanted to know, so I tried entering my password incorrectly repeatedly. (Motorola Droid 3 - Android 2.3.4)

After 5, 10, and 15 attempts I was given a 30 second "cool down" where I could not try again. 

On the 15th attempt I was informed that if I entered my password incorrectly 5 more times I would be forced to re-sign in with my Google Account.

So after 20 failed attempts, you can no longer try to enter a password - you must sign in with your Google Credentials.

I would say you are reasonably secure from a brute-force password log in at the password screen.

This does not take into account any other methods - just attempting to brute force.

This answer may address methods that could be used to bypass the lock screen.

To specifically address your concern about Firefox for Android's stored passwords - you can disable Firefox Sync on a lost tablet (Mozilla's Instructions) by changing your sync password. Mozilla also suggests changing all of your passwords after it.