This is, and is not, connected to the prior passage. We’ve moved past the parable of the vineyard and invoking the now famous image of a light under a basket. Let’s start by noting that this section ends with the warning: text unnecessarily obfuscated.
One of the famous conceits of Rome was that it was the “Light of the World.” In their world-view, the application of Roman civilization upon a place was the effect of the Spirit of Rome illuminating those peoples with its light. This was a standard element of Roman patriotic writing, frequently used and confidently invoked.
The lamp in this parable could represent the Light of Rome that isn’t to be hidden, but placed on high to enable progress. It could also mean that the illumination provided by the Gospels should be widely spread, not kept secret.
Or it could just be a rude dig at the Hebrews who kept their holy scrolls under baskets or in shelves or under their beds, where the lamp indicates spiritual illumination and the Judeans were being mocked for hiding theirs.
The next phrase ties in with having “ears to hear”, so let’s look at this more closely. Nothing is hidden, except to be revealed – would mean that things at this time of Jesus were obscured, but would be obvious in due time. Given that he is talking about the destruction of Jerusalem, this is true – it was inconceivable that Rome would have destroyed the great Temple in Jerusalem, until they did it. This is no longer hidden, and it wasn’t at the time the Gospels were written.
It appears that the phrase about having “ears to hear” was written as a clue to the reader, because it would have at best been a reminder to those at the time that they were being fed nonsense. These words about having ears to hear and eyes to see were put in the mouth of Christ to alert the reader of the Gospels that an important message was just given. It was something clear to the Roman reader, but hidden to the 1st century Judeans.
Mark 4:25 is a commonly quoted passage, but its construction is vague enough to fit many agendas. “Take care what you listen to” is an odd command at the heart of a series of parables, although it pairs well with having “ears to hear”, connecting this passage with the prior one about the lamp under the basket. Take it as a given that the prior verses were comparing the healing illumination of Roman culture as opposed to the hidden and shameful Judean culture. Then we can take “Roman culture” as the “it” here and the passage makes complete sense in the context of the “Rome is better and will crush you” theme that’s gone along so far.
Giving us a contextual translation of 4:25 as: “Those who have Roman culture will be given more (when Rome takes this state over). Those without will be rejected.”