Mac mini 2014 4k + 2k Monitor Configuration
It seems, from the spec you've listed, that it should support one 4K display via HDMI at 30Hz, and another display via Thunderbolt.
However, everymac.com does not list the HDMI options beyond 1080p at 60 Hz.
For a 2014 MacBook Pro with the same Intel Iris 5100, everymac does say:
This model supports a simultaneous maximum resolution up to 2560x1600 on two external displays via Thunderbolt. Alternately, it can support a single display up to 2560x1600 via Thunderbolt and a single 1080p display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160 at 24 Hz via HDMI.
... which tallies with the similar spec from Apple for the Mini.
I'd also draw your attention to the fact that even the 2.6 GHz i5 CPU is only a dual-core, and was considered quite 'light-purpose' at the time.