No need for me to install an SSD drive on an HTPC...The internal 500GB HDD works fine...I wouldn't be able to use the additional performance for anything...I don't think going to 8GB would improve performance either for a machine that only rips and plays back movies.
Have you ever used a Mac with 8GB of RAM and an SSD as a boot drive?
You might feel the way you do at the moment if, as you say, you plan to just rip and play back on a single display, but if that Mini hosts your iTunes library and you do iTunes home sharing with other devices, or if you start to multi-task or record HDTV or transcode those rips or serve files to other devices while you're also using the Mini for playback in the home theater, believe me, both of those upgrades make very real differences. Just ripping and just local playback, I agree with you, 4GB and your stock 5400rpm drive are fine: I found the 5400rpm drive plenty fast for my toughest blu-ray rip and ripping isn't challenging in the least system resource-wise, no amount of RAM or CPU will speed up a rip-locked external blu-ray drive. MakeMKV is going to take roughly the same amount of time with whatever RAM or drive you have.
But, it costs less than $40 to go to 8GB of RAM and everything
about OS X and iTunes improves afterward, everything is more smooth and efficient. I understand if there's some trepidation about opening up your still under warranty Mac to install an SSD, if that's the case try booting a 2011 Mini off an SSD, even over firewire 800, it is noticeably superior to the internal 5400rpm drive if you ask anything of your machine beyond mere rip and playback. And booting that Mini off an SSD over Thunderbolt? OMG as the kids like to say. (If you do go SSD, since there's room for two drives inside, it's a good strategy keeping the stock 5400rpm drive as a bootable clone of your system.)
Don't for a minute think your HTPC experience wouldn't significantly benefit from doubling the RAM or booting from an SSD; do both and you'll wonder how you ever managed previously.