The Euro 3 regulations came into force July 2007, these mainly consisted of much stricter emissions regulations and some detail electrical changes.
The big upshot was the death of the two-stroke motorcycle, which other than a few mopeds could not pass the Euro 3 limits.
The other thing to change was that headlamps now had to be permanently on.
The Euro 4 regulations came into force January 2017, this mainly consisted of even stricter emissions regulations and compulsory changes to braking systems.
This means that all machines will now be fuel injected. The really significant changes are the braking systems. All machines below 125cc now have to be fitted with either ABS (Anti lock braking systems) or a combined braking system. Machines over 125cc now have to have ABS.
This is a huge simplification of the rules and regulations, the full specs can be found at http://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX:32002L0051 and http://transportpolicy.net/index.php?title=EU:_Motorcycles:_Emissions#Euro_4_and_5
So what does this mean, right now?
It is not possible to make such a huge change without allowing some cross over time. The powers that be, have allowed a number of machines to be listed and given up to two years to be sold and registered. Any Euro 3 compliant machines not listed had to pre registered before the end of 2016.
So right now there are three choices theoretically available for brand new machines.
A pre registered Euro 3 bike, these may get cheaper as the 1st of March approaches.
A derogated or end of series Euro 3 bike, these have the advantage of being pretty much at last years prices, so there are savings to be had!
A Euro 4 bike, these will be cheaper to run, thanks to the lower emissions limits and hopefully more reliable with the EFI systems. The braking systems should hopefully make the bikes safer, particularly for new riders. The extra technology will increase prices but the savings in economy and safety should make up for it.
All these changes are only relevant to brand new machines and do not affect any currently registered or older machine.