How do you respond when someone purposely ignores you?

Before you decide what to say, look at how long it has been and what her last few messages looked like. Four days of silence after a message where she answered every question you asked is different from four days after she sent a short, flat reply. Reading her last message as a whole matters more than the one you are about to send. If she was warm and then went quiet, the most likely story is that something came up or she lost momentum. If she was never very engaged, the silence is probably her answer already.

Also check whether you have been the one doing most of the work in the thread. If every question, plan, and joke has come from you, the silence may be her way of saying she is not carrying her half. That does not mean the conversation is dead, but it changes what a useful reply looks like. It should be easy to answer with a yes, a no, or a time, not a paragraph.

A reply that fits this situation has a clear job. One kind offers an easy out: a light, single-question message that makes it cheap for her to answer, with no pressure about the silence. The draft should be one line, end on something she can say yes or no to, and leave out any mention of the gap. A second kind closes the loop: a short message that tells her you are fine either way and are not planning to keep asking. You can tell a draft is doing this job when it reads as finished rather than hopeful. It should leave out apologies, explanations of why you are reaching out, and anything that asks her to justify the silence. A third kind is no message at all, which is the right move when the thread has clearly run its course and the next thing you say would only be a reminder that she has not answered.

Timing is where most people undo a decent message. Send your one follow-up in the early afternoon or early evening, when people are usually free and not in bed, and avoid late-night messages that read as needy or as a bid for attention. Wait at least two full days after your last message before you follow up, and longer if she was clearly busy that week. If she has not replied to a single follow-up, do not send a second one within a few days. A second follow-up is only worth sending if something new has happened, like a plan that has a real date on it. If you have sent one message and waited, and then still have nothing back after another week, stop. Do not send a farewell message or a final check-in. The silence is the answer, and continuing to write only makes the thread about the gap.