Lascaux IV tickets

We were there last year in late April and bought our tickets from the TI in Sarlat a couple of days in advance. Some English tours were sold out for particular hours. If you're tied to one particular day you may want to buy them before leaving to be safe as May could be busier than April was last year.

I have visited the site twice and each time bought tickets for English tours that day. You don't consult with the guide about tickets or not -- you just get tickets. Once we got them in Montignac at the ticket office for Lascaux; once tickets were only available at the site. I don't know which is the case now but we have never had trouble getting them for a tour that day.
Because COVID changed so much, you may want to check the website and see if ticket are available day of or a day before and if not book earlier.

I just purchased my tickets 20 minutes ago for an English tour in May. I believe there were something like 26 places left for the English tour on my chosen date. Easy enough to do online (surprise, surprise). If you know which day will be your free day and you know that you want to visit Lascaux IV, I would say buy them in advance. The benefits of getting tickets in advance are 1) you know you've got the tickets for an English tour on your chosen date (all English tours seem to be at 11:10 AM), 2) there is no last-minute stress about something possibly going wrong (Murphy knew a thing or two) and 3) I, for one try to avoid spending my vacation days on a computer/phone booking are trying to book last-minute tickets. Have you considered Font de Gaume? That would be my 1st choice. It's a real cave with the real paintings. I was there years ago and it was the most fabulous experience. I know that tickets are harder to get for Font de Gaume but I think it's well worth it if you can purchase them in advance.