Resignation Letter Help - Unsure of Last Day

I was recently offered a great job in a different state and will be starting in August. My current employer has asked for four weeks notice; I am considered at will. I would like to provide four weeks notice but I'm not sure how the timeline of moving is going to look with my current lease up in 3 weeks and how quickly my new job's relocation services work. I need to give notice ASAP, but do not have a "last day" nailed down. How should I address that in my written letter of resignation? I'd like to meet with my employer this week and present them with a written letter at the meeting. Thank you!

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