The quick answer
March through May and September through November are the best months. You can watch baseball, football, basketball, and volleyball all at once.
Month-by-month breakdown
January-February
B.League basketball and SV.League volleyball are in full swing. Baseball is in spring training (exhibition games from late February). J-League hasn't started yet. Cold weather keeps you indoors, which works well for arena sports.
March-May
Everything overlaps. NPB baseball opens late March. J-League starts in February and is in full swing. B.League and SV.League are heading toward playoffs. Cherry blossom season (late March to mid-April) plus live sports is the ideal combination.
June-August
Baseball and J-League continue. B.League and SV.League are finished for the season. June is rainy season, so outdoor stadiums are a gamble. July and August are hot and humid. Dome stadiums (Tokyo Dome, PayPay Dome, Vantelin Dome) are comfortable regardless.
September-November
Baseball enters the pennant race and playoffs. J-League is in its final stretch. B.League and SV.League open their new seasons in October. The weather cools down, making outdoor games pleasant again. This is the second-best window after spring.
December
Baseball is finished. J-League wraps up. B.League and SV.League are active. The Emperor's Cup football final is held on or near January 1.
The overlap sweet spot
Late March through May: all four sports are active simultaneously. A one-week trip in April can include NPB baseball, J-League football, B.League basketball, and SV.League volleyball. No other country offers this much variety in one visit.
Buying tickets
For all sports, buy through official team sites or English-language platforms. B.League and SV.League tickets are easier to get than baseball and football.