## header file
NSUInteger numTimes;

## implementation file
- (void)viewDidLoad {
  [super viewDidLoad];
  ...
  [NSTimer scheduledTimerWithTimeInterval:4 target:self selector:@selector(myMethod) userInfo:nil repeats:YES];
}

- (void)myMethod {
	numTimes++;
	//NSLog("CallsToGPS: %d", numTimes);
}