I think if you feel the need to physically discipline your child, there's no reason it should ever be hard enough to leave a mark past a couple minutes.
Blood and bruises isn't going to teach that kid a lesson he couldn't have learned without them. And at 4 years old, because a child that young doesn't/can't always correctly connect the dots, you may be mistakenly teaching them to take their anger out by hurting others. A 4 year old might not always get "I did x, so they did y", they might get "dad is mad, when dad is mad he does y, so when I get mad, I do y".
Blood and bruises isn't going to teach that kid a lesson he couldn't have learned without them. And at 4 years old, because a child that young doesn't/can't always correctly connect the dots, you may be mistakenly teaching them to take their anger out by hurting others. A 4 year old might not always get "I did x, so they did y", they might get "dad is mad, when dad is mad he does y, so when I get mad, I do y".