Always ask the lady you wish to use as a reference if she is OK with providing it.
If you are not a regular, or there is a chance she may not immediately remember you, make sure you provide sufficient details in a single and initial correspondence. After all, she is doing you a favour, so don't make her jump through hoops to try and remember your time together.
Something along the lines of.
Hi Jill. My name is Joe Smith. We have met 4 times in the past, most recently May 23rd for 2 hours at the hyatt. My phone number is xyz and my email address is xyz.
I am interested in seeing ____________. Her website is ______________, and she has requested a reference. Would you be so kind as to provide me with one.
You may also remember me from our conversation on quarks or the paw print of my dog that I had tattooed on my shoulder.
I had a great time with you, but variety is the spice of life, so I hope you will provide me with a positive reference.
Thank you