Whether to stay or leave is obviously an important decision that can impact your career, and you may not want to base it on something as simple as the things you've mentioned here.
Of the things you mention, there seem to be two primary concerns.
One is that you definitely won't get a raise until a new round of funding comes in, and even if that funding does arrive I assume a raise is not a guarantee. We obviously aren't aware of your status or current and past performance. We also aren't aware of how much you are making now, and whether that amount is perhaps at the higher end or lower end of market rate already.
Two is that the company seems to be having at least some financial concerns.
You've also told us "they anticipate that the round will close over the next couple of months, if not weeks". That sounds optimistic. Being that the next salary review isn't for a couple weeks, it sounds as if this entire thing could be about nothing.
If you like your job enough where not getting a raise is OK for at least a few months, you should stay.
Beyond liking your job, are you still learning? Are you becoming better at what you do year after year by staying there? If the answer is "no" to either of those questions, you should probably leave regardless of salary raises and free champagne.
You should always be prepared for the worst, of course, so if an event (layoff, closure) were to happen you should always want to be in a position to start looking for work quickly.
I don't think suggesting a 4 day work week due to an incomplete salary review is a good option. That suggests that you may have anticipated a 20% raise, which isn't all that realistic. Making unrealistic demands of a cash-strapped company could be a quick way to a layoff.
I'm not sure at this point it's even relevant to ask the founders any questions. They seem to be forthcoming about things - it's not like they told you there will be no salary reviews the day before they were scheduled to happen, and if they have been trustworthy in the past you should have no reason to doubt their honesty now about their expected timeline for the next round to close.
When should one start looking for jobs? There are lots of reasons. If you are not fairly compensated for what you do and there are no other 'upsides' to your employment that make staying a good option. When you aren't learning or challenged by what you do. When you feel work is having a consistent negative impact on other parts of your life. When your company clearly makes decisions that do not consider the best interests of their employees.
Good luck to you.