On the Mikrotik download page, download v3.30 “combined packages http” and ftp it to the router. You must go to 3.30 first to get the free upgrade to v4
Reboot.
Telnet (or winbox new terminal) and run this command:
/system router upgrade
Reboot.
In Winbox, go to LICENSE and choose UPGRADE LICENSE.
Reboot.
Download version 4.2 like you did with 3.30
Upload it over ftp (or open FILES in winbox and drag it in that way)