Nonetheless, whether we take your number of 4,500 or the 8,257 quoted by the USSBS, knocking out a couple of dozen of the largest plants would drop total generating capacity 20% or more and leave the entire grid even more dangerously overextended and vulnerable to disruption.
It sounds like it would have been worth trying - especially as these are the sort of relatively fragile high-value point targets that get the Mosquito Mafia on this board all hot and bothered.
The problem is that the size of plant you're thinking of just didn't exist - as mentioned by Rast, Germany (or indeed pretty much any other country) didn't have a national grid yet, so the size of a power station was limited by the distance of the wires stretching from it. The only power stations that might be big enough to make a difference are already going to be in cities and within the bombing area anyway - for example take a look at the list of power generating companies for London before nationalisation (lifted from Wiki):
Barking Borough Corporation
Barnes Borough Corporation
Battersea Borough Council
Beckenham Borough Corporation
Bermondsey Borough Council
Bethnal Green Borough Council
Bexley Borough Corporation
Bromley Borough Corporation
Dartford Borough Corporation
East Ham County Borough Corporation
Erith Borough Corporation
Fulham Borough Council
Hackney Borough Council
Hammersmith Borough Council
Hampstead Borough Council (1894)
Ilford Borough Corporation
Islington Borough Council
Leyton Borough Corporation
Poplar Borough Council
St Marylebone Borough Council
St Pancras Borough Council
Shoreditch Borough Council
Southwark Borough Council
Stepney Borough Council
Stoke Newington Borough Council
Walthamstow Borough Corporation
West Ham County Borough Corporation (West Ham Electricity Board)
Willesden Borough Corporation
Wimbledon Borough Corporation
Woolwich Borough Council
Central London Electricity Limited (formerly Charing Cross Company)
Chelsea Electricity Supply Company (formed 1886; taken over by Charing Cross Co 1937)
Chislehurst Electric Supply Company
City of London Electric Lighting Company
County of London Electric Supply Company
Foots Cray Electricity Supply Company
Hampstead Electric Supply Company Ltd (records from 1898) †
London Electric Supply Corporation (LESCo) - formed in 1887 out of Grosvenor Gallery Electric Supply Corporation, London's first commercial electric power supplier
London Power Company
Notting Hill Electric Lighting Company
South London Electric Supply Corporation
South Metropolitan Electric Light and Power Company
Nearly all of these will have had their own coal-fired power stations, although in the case of London there was some consolidation in the 1920s which led to many of the smallest power stations being closed - the bigger ones which remained seem to have been on the size of tens of megawatts, rather than the hundreds required for any one plant to be supplying 2% of national electricity. It should be noted, indeed, that all of the largest power stations in the world at this time were hydroelectric - and even they were generally pretty small outside North America or the Soviet Union. Operation Chastise, for instance, knocked out 5.1 MW of generation.
So while you could make a major impact on German production by taking out power stations, you aren't going to do it in one raid. Instead you're going to have several squadrons of Mosquitoes spending their time flying across Europe plinking away at little tiny power stations...