You head through the door. Immediately, you are blinded by a sudden burst of light. You stand in the doorway, blinking, as the door closes and pushes you into the room. As your eyes adjust, you see that there are four bright lights in front of you, shaped like the sun. You look closer...
Given the mouseover text,
It's not clear if that's supposed to be us talking to ourselves thinking through what to do with four sun pictures, or if we're reading something we see when we examine more closely, or what. My best guess, after OP points out in a comment that Solis is shortened to Sol, is that we're supposed to read the mouseover text as our own musings, though what inspires them is completely unclear.
Given that, ...
Sol x four — latin Sun (shortened) x 4
Sol x fur — remove the 'o'?
Sul x fur — turn the 'o' into a 'u'?
looks an awful lot like
Sulfur or Sulphur
which seems to be in keeping with the other puzzles in the series which have all been alchemical looking substances, spices, or elements.
This might probably be off track but is the answer related to?
with each Sun depicting the four features : the Sacred Spring, the Roman Temple, the Roman Bath House and the Museum.
The image description says "Latin? Remove the 'o'? Aren't you supposed to turn the 'o' into a 'u'? I'm confused"
So, in Latin sun is Solis.
If we do not remove the 'o' but turn it into 'u', we get Sulis, the goddess who fed spa at baths. She is displayed at Roman baths, so that seems a plausible answer!? p.s. am still unsure.