Google 'investing minimising regret'. I think you've had some good advice already on this thread, but at the end of the day this is what it's about for most of us.

I don't think it makes sense to sweat too much about the minor details. For most - but I suspect not all of us - changes in Bitcoin price are utterly unpredictable. It may be $4000 or $40000 at the end of the month. You can't predict it, so if you fail to predict it you shouldn't beat yourself up about it. And as a consequence the time to invest is always 'now'.

Personally I think the psychology of how to avoid regret is more important than the question of when to invest.

If bitcoin goes up to 40k and you haven't bought in, how will you feel?
If bitcoin goes to 0 and you have put in too much, how will you feel?

I've watched one of my poor colleagues recently see his Bitcoin investment rise in value from $1000 to $2000 then drop back again, at which point he panicked and sold the lot at no loss or gain to himself. He was checking the price every hour and feels much better now he's been rid of them.

If Bitcoin continues to appreciate, 'when' you bought into Bitcoin can be less important than it first appears. Sure, if you'd bought a few weeks ago you could have bought twice the amount of bitcoin, but assuming Bitcoin continues to appreciate then all this means is that you achieve whatever investment goals you have slightly later. For example, I'm about to liquidate the bulk of my Bitcoin in order to buy a house. If I'd had 10x the amount of bitcoin, I'd have been able to buy my house last year instead of this year. So how much do I now regret not buying 10x the amount of Bitcoin back when they were a few tens of cents each? Not a lot, really.

The more complicated question is how to sell Bitcoins that you have seen rise in price, in order to minimise regrets should they suddenly tank. I HODLed mine until they were ripe to pay off the house all in one go, but arguably a strategy of selling them off gradually would have been more sensible. We're mostly Bitcoin Bulls on this forum, but a total crash remains possible.