You can use
WebViewExtras and it's clearCache method to clear the WebView cache.
The WebView uses an SQLite database to manage cached items.
The cached items are saved with meaningless names and the database contains a table with the meaningless name and original URL (as well as many other columns).
So to make use of the WebView cache you'd need to access that database to find the meaningless name that contains a cache URL object.
I'm not sure if your B4A app has permission to access that database.
It looks as though you can force Android to use the device external memory for all of it's cache objects:
Saving the Android WebView cache on the SD card – devahead BLOG
Note that will use the external memory for
all application objects not just the WebView cache.
For Android versions greater than 2.1 it's possible in B4A to sub-class the Android Application class and override it's getCacheDir() method.
But for Android version 2.1 and earlier it would be required to override the Activity getCacheDir() method (not the Application getCacheDir() method) and that, as far as i know, is not possible in B4A.
In B4A we can sub-class the Appplication class but not the Activity class.
If you'd like to try moving the cache to external memory then let me know - it'd require a quick n simple library to be created.
Martin.