Suspended for Day Trading suspended my account for 90 days for day trading.
My account balance never dropped below $35,000.
I do not understand.

Any amount used on open options or futures trades is deducted from your account balance to calculate your account balance applicable to stock trading for the purpose of the day-trading min $25,000 requirement.

This happened to me several times at IB on an intraday basis, I never had any issue having them lift the restriction given the cash account balance at the time of my call.
OK for example,
I have $50,000 in cash in my account.
I buy 1000 shares of an ETF for $60,000.
I then sell 1000 shares of the ETF for $61,000.
what happens?
Have you read the Pattern Day Trading (PDT) rules?
May be a silly question, but I'm assuming that this is a "margin" account you are trading? If so, then you're fine for day trading under the Pattern Day Trading criteria (these accounts require a minumum of $25k and NOT dipping below that at the close of the trading session with all trades closed out).

If you are trading equities, there should not be a problem. But if you are trading other tradables out of the same account (refer to Dom's post), then the way the brokerage tracks your equity is obviously affected. Usually, there is not an immediate suspension of trading on the account with the first "infraction." Then again, it depends on each individual brokerage's policies. If you could provide more info on the trading activities and what/why they told you your account was suspended, then maybe there's more feedback we can provide here.
I just opened an account at with $40,000.
Free trades for 30 days and $3 per trade thereafter.
Let’s say that I do 4 day trades in one week.
Then I would be identified as a Pattern Day Trader.
What would happen after that?
I'm gonna assume this is a margin account. I'm also gonna assume you're trading equities only. In addition, I'm gonna assume your account didn't dip below $25k. Gonna also assume you didn't get a call for exceeding your 4 to 1 margin for trading (the execution platforms out there almost always are set up to prevent you from sending out an order that would allow this to occur).

Based on your prior post, as long as you're just trading equities, you could have traded 4, 5 or a Bazillion "round turn" day trades and everything would be fine.

The question is: what else have you traded in the account besides equities and/or did you hold a position into after-hours trading or overnight? If you in fact received a margin call and did not meet it within the specified time frame, your account would be frozen by your brokerage firm.

If you could share some answers to the questions asked about your account and trading activity etc., I could help ya better nail down what happened. There's always the possibility of a mistake in the back office (or back office software) of the brokerage firm or even a goof during clearing ... very unlikely though.
I traded only stocks.
I did hold equities overnight.
Pre and post mkt (after hours) trading isn't a big deal (considered same day), but overnight holds are and affect your margin and "day trading buying power" (DTBP). All I can guess is that perhaps you got a margin call that you didn't meet within the specified time frame by transferring in funds/equity.

What did your broker tell you specifically in y'all's communications about what led them to suspend your account?