The plugin allows for anyone who owns a hacked PSVita to download any type of files from the PS Vita's built-in web browser, like you would with any regular web browser. The feature is originally restricted to image and video files on Sony's official firmware. Each file downloaded with the plugin will go into the ux0:download folder of your console. The plugin is also available for PSTV.
Download Enabler was created by TheFl0w, who is also the creator of other PSVita plugins such as VitaShell, a file manager for the HENkaku homebrew enabler and NoNpDrm, a plugin to bypass any DRM on the console.
The fourth version of Download Enabler seems to only add support for the 3.65 and 3.67 firmwares. Although there is no current way to hack a PSVita beyond firmware 3.60, tricks have been found to add minimal things such as the ePSP VHBL. This only means that TheFl0w is getting close to releasing HENkaku/taiHEN for firmware 3.65, as previously promised.
Consequently, this new version will most likely not change anything for a lot of people, given that it's not possible to hack your console on firmware 3.65 and above... yet. We are still patiently awaiting the release announcement for Henkaku/taiHEN for 3.65 firmware.
The plugin is available for download from TheFl0w's github, and you can install it like any other PS Vita plugin, given that you're running a PS Vita or PSTV with firmware 3.60.
We will keep you updated for any announcement from TheFl0w regarding HENkaku/taiHEN for firmware 3.65.