Just an official confirmation on Ken's advice for you...
545541 - 1768-L4xS does not appear on Studio 5000 v21 hardware list
Access Level: TechConnect
To download RSLogix 5000 versions, previous to v21 (Logix Designer), go here...
Software Downloads
...and select "Software Downloads by Serial Number".
Use your Studio 5000 serial number/Company name to validate.
This will usually take you straight to a download page for the latest version of Studio 5000. This will be displayed with "Latest" appended to the end of the description...
Example: 9324-RLD700NXENE Studio 5000 PRO EN
Latest
You will notice under the "Version" drop-down list that the only option available is the latest version.
Just beneath that, click the drop-down list indicator to the right of the "Change product shown above here" box.
Select the equivalent edition that matches your earlier product, but that has "...Formerly RSLogix" appended...
Example: 9324-RLD700NXENE Studio 5000 PRO EN-
Formerly RSLogix
The page will update with the above displayed as your new selection.
The "Version" drop-down list will now allow you to select whichever previous version of RSLogix 5000 you require. Select the version that matches your 1768-L43 processor's current or intended firmware major revision number and download.
You can go back and do the same if you require any other previous versions.
Regards,
George