what-all is the highest platform number when the frequency (per hour) is 4, the operator is london overground and the destination is west croydon?
Platform Frequency (per hour) Destination Operator Line
Honor Oak Park railway station
1132489
real real text text text
2-1132489-1
Services
Services
1 4 Highbury & Islington London Overground East London
1 4 Dalston Junction London Overground East London
1 4 London Bridge Southern Metro
2 4 Crystal Palace London Overground East London
2 4 West Croydon London Overground East London
2 2 London Victoria (Mon-Sat) Southern Metro
2 2 Caterham (Mon-Sat) Southern Metro
2 2 West Croydon (Peaks & Sun only) Southern Metro
2 2 Tattenham Corner (Sun only) Southern Metro
SELECT MAX Platform FROM table WHERE Frequency (per hour) = 4 AND Operator = london overground AND Destination = west croydon
4 london overground west croydon