If you make 4,800,000 円 a year living in Japan, you will be taxed 1,222,980 円. That means that your net pay will be 3,577,020 円 per year, or 298,085 円 per month. Your average tax rate is 25.5% and your marginal tax rate is 34.4%. This marginal tax rate means that your immediate additional income will be taxed at this rate. For instance, an increase of 100 円 in your salary will be taxed 34.39 円, hence, your net pay will only increase by 65.61 円.
A 1,000 円 bonus will generate an extra 656 円 of net incomes. A 5,000 円 bonus will generate an extra 3,281 円 of net incomes.