Yup, plastic saddles can get more flexible over time and can creep to sag 'a bit'. Usually it takes a fair few years and miles though. Most folk don't notice the change as it happens so slowly. It is unlikely that your saddle has changed so drastically so soon. If you are having problems in <4000km, it is probably because of your bum or something else changing, not the saddle.
Brooks comment: After almost exclusively riding B17 saddles of various types for a couple of decades, I'm going through another saddle trial phase. The durability just isn't there any more, because of younger cattle, less durable tanning methods and less care by Brooks regarding orientation of leather grain. For what it is worth, for me the organically tanned Select models last about 3x as long as the modern Standard models. The 1970s-era Brooks saddles I've used seem almost indestructible. YMMV
You didn't ask for specific recommendations and what works for someone can be the worst possible option for somebody else, so ignore the following. The Rivet Pearl is the best match for me so far and has lasted exceedingly well for HK, lots of miles and rain since 2012. She does about 18,000 miles a year using about 4 bikes. Her Brooks B17S Imperial equivalents lasted less than 6 months each. Some people like to ride hammocks but we don't.
HK reminds me that there can be incompatibilities between specific combinations of shorts/ pads and saddles. Is this a possibility, have you started using a different model of shorts/ pad recently?