Has happened to me as well... unfortunately during this Black Friday weekend as well I might add.
After an long pointless message with "Paypal Help" and some Google-Fu basically this -
Help desk will spout out the same reasons why Pay by 4 won't appear... they will say is Pay by 4 is not available on all stores and each purchase is subject to "approval" but you should just choose the normal full payment with Paypal. Which if you have been using Pay by 4 regularly will seem like rubbish and the added just use normal payment brilliantly condescending.
After asking them to clarify further - "approval" can basically mean that at any given time you will not be allowed to use Pay by 4 because it's not available in your country, the shop doesn't have pay by 4, bad credit or suspicious activity. If any of those happens on your account your Pay by 4 will be revoked until your account gets "approval" again after a certain period (some folks on google say it comes back in 2 weeks.. maybe more). Problem is Paypal Help will NOT tell you what caused your account to have the Pay by 4 option revoked and will constantly just gently remind you that hey you can just use full payment! (Which defeats the purpose of actually split payment options...) So if you have been a regular user of Pay by 4 and it's stopped appearing even in stores you've previously used it on its probably most likely the "Bad Debt" or "Suspicious Activity" reason... but since Paypal HELP won't actually "Help" to tell you the reason to try and contest those reasons your only option is to really wait it out till Paypal decides you are a good kid and return Pay by 4 again till they arbitrarily remove it again..
Edit: Also forgot... there have been times before when Pay by 4 system was actually down and not available. With a fair few folks the last few days saying the same thing this might also be a credible reason. Of course sadly this is speculation since as I said above the "Help" people are basically just guys copying and pasting script replies.
