It depends on where you are. In North America, NEMA standards have been taken over by UL, so instead of being self-regulated standards, they are now tested and approved by a 3rd party NRTL (Nationally Recognized Testing lab).
UL Type 4 is indeed "wash down" because the test for it is water applied at 65GPM from a 1" nozzle from ANY angle, with no penetration of liquid.
UL Type 4X is the same liquid tight standard, it just adds a degree of corrosion protection against chemical agents, usually cleaners. Which chemical agents is a matter of reading test data, because not all Type 4X systems are required to be impervious to all chemicals, they just have to be clear on WHICH ones in their data sheets and it's up to the user to confirm applicable requirements. But in general, salt water is inclusive in all versions. Type 4/4X is also approved for "outdoor use" which means it protects against ice, snow, dust, sunlight etc.
OUTSIDE of North America, there is a different rating system based on IEC (International Electrotechnical Commission) standards, using what is called a "degree of Ingress Protection", abbreviated as "IP", followed by a 2 digit number. The first digit denotes the degree of protection against mechanical touching and particle entry, ranging from 0-8, with 0 being essentially "open" so it can be touched, 8 being essentially bullet proof. Nobody really makes anything greater than IP6_, which is fully protected from contact and is also dust tight. The second digit relates to the amount of protection against moisture, ranging 0-8 with 0 being unprotected and 8 being total continuous submersion in (I think) 10 meters of water (40'). There is however no exact direct equivalent to the NEMA hose test in the IP system. Most people interpret IP_5 as being "water tight", but the test standard is essentially like a gentle shower head spray only from the top. Hose directed water is NOT necessarily excluded. IP_6 is referred to as "splash protection" or "heavy seas", meaning an occasional short burst at relatively higher force than IP_5, but still not a continual attack by a fire hose. IP_7 is rated for actual submergence for short periods, but at 1m depth, for no more than I think 3 minutes.
So IP65 is essentially equivalent to what we would call Type 12, which is "drip tight" and dust tight. I would not consider it "washdown" and many inspectors here in North America will not accept that for food processing machinery controls, where cleaning procedures involve hosing the machines down with a fire hose spewing caustic cleaning solutions. The closest you will find to type 4X, and often acceptable, is IP67. Just be aware that if Joe Bubba the Cleaner points his fire hose at an IP67 push button for 5 minutes to get that bit of slime off of it, it might fail because it wasn't really designed to deal with that.
In reality, most of the major players in the pilot device world, even those from the EU, will test their devices to BOTH standards. That's because for example if you pass UL Type 4X, you will definitely pass IP67. So they basically test to the hardest standard and attain both ratings (IP ratings are self-certified anyway, no 3rd parties involved). Where you get into trouble is when you buy over the internet from bottom feeder suppliers who make any kind of claim because they don't have to back it up.
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