You'll be fine, the first few days you'll feel tired but the natural energy boost will soon kick in and you will probably feel more ready for work than ever before. 4 hours a day isn't very much, even if it is strenuous work- so I can't see it affecting your performance. Be sure you relax properly after work by doing yoga, having a bath or just listening to music. That way you're more likely to have a good full night's sleep and feel ready for work the next day. I know it's very basic advice, but I swear it works!