Galaxy s4, damaged "lcd" and screen locked. Tethered G s3. Is it possible to access G s4 files using working G s3 ?

SOURCE: how to brighten the screen during internet usage with galaxy s3

While in browser, open settings (bottom left button) and change display setting. The s3 has separate display settings for browser and general phone. Hope this helps.

SOURCE: Can i change my (l747) galaxy s3 lcd screen for another sgs3 lcd screen for example (l710) screen? are the screen compatibles?

Yes, they are compatible. You can see the pictures in the following, exactly the same.
http://www.etradesupply.com/samsung-galaxy-s-iii-sph-l710-lcd-screen-and-digitizer-assembly.html
http://www.etradesupply.com/samsung-galaxy-s-iii-sgh-i747-lcd-screen-and-digitizer-assembly.html

SOURCE: Cracked galaxy S3 screen

Yes the screen can be fixed just take it to a repair shop such as a person who specializes in samsung galaxies

SOURCE: Trying to use ATT Motorola Atrix phone with my Tmobile service and Galaxy S4 Sim card

You can get the the network code or APN on the t-mobile website.

SOURCE: My Samsung galaxy s4 has stopped working. I tried

Try taking it back to where you purchased it, they may be able to get it loaded.