In the analogy "Hard:Soft::Intermittent:," we can see that "Hard" is the opposite of "Soft." Therefore, we're looking for a word that is the opposite of "Intermittent."

Option 1: "Incessant" means continuous and unceasing, which is the opposite of "Intermittent." This is the correct antonym.

Option 2: "Inadequate" means not enough or insufficient, which is unrelated to the concept of being the opposite of "Intermittent."

Option 3: "Indifferent" means having a lack of interest or concern, which is also unrelated to the concept of being the opposite of "Intermittent."

Option 4: "Sporadic" means occurring at irregular intervals or infrequently, which is a synonym for "Intermittent." This is not the opposite.

So, the correct answer is Option 1: "Incessant."