"It's four o'clock."
Where have you seen だある? That sounds ungrammatical to me. Also, formal is a way broad term and Japanese with its system of speech levels makes formality a complex issue. ある is the casual form of あります, so... it's confusing. である is a literary form pertaining to scientific writing. Scientific writing contains formal expressions but again, formal here is different than formal in relation to honorific speech for example. I don't mean to be rude.
Considering politeness (which is an expression of psychological distance, i.e. whether or not you're close friends with someone) we get (from least formal to most formal): よじ/ よじだ/ よじです。Some linguists consider yoji. an ungrammatical colloquial expression as it has no verb, and a verb is the core of a Japanese sentence — a sentence with just a verb is just fine, but one without, not that much considering formal linguistics.
I think it has not been introduced as a lesson yet. It has been introduced in this discussion in answer to someone's question. There are some advanced students in these discussions, so you will probably see alternative ways of saying things that we have not learned yet and will come up in later lessons. I don't worry about having to learn these now. I just read the discussion and focus on the present lesson, otherwise it would be overwhelming for me.
The forum is for extra discussion. Without it the course should be full still. I know it's still a new course and there can be issues, so I just wanted to flag it that there are new words used in the course without ever introducing them which is not how all the other courses work.
it turns out there are others options available to build a sentence, but they haven't previously shown up. i had the same thing happen, only when looking here did i discover using the blocks available i could have made an alternate sentence to what i'd been taught, that would also have been correct.
De aru is quite frankly a written form. I don't see it having that much to do with formality in the same way as desu/ masu. Desu/masu style is addressive — it acknowledges a listener. De aru is declarative. It sounds confident and firm and is used when writing research papers etc. nowadays. Strictly, speaking it's not addressive and if it had to be it would become de arimasu. Formality, I think is more apllicable to honorific speech keigo (the Japanese consider desu/ masu keigo too). Desu/ masu I'd rather call polite speech. Hope that helps someone. Sorry if I sound cocky, I just major in Japanese.
I apologize in advance for ranting.
I'm try to practice speaking this, so I'm using voice-to-text. WHY DOES VTT KEEP MAKING NUMBERS THE ROMANIZED CHARACTERS RATHER THAN HIRAGANA OR KATAKANA!? Everything else in the sentence is Japanese, but let's not make the numbers Japanese. BECAUSE THAT MAKES SENSE!