just calculate it out
[.5 x bore(in mm)]^2 x 3.14 x stoke (stock is 57mm, +3 is 60mm, +4 is 61mm)
now multiply by 1000 and that's your new displacement
strokers don't actually add that much to the cc's but you get better torque. exactly how much depends on what else you do at the same time. the best option is to have it ported at the same time to optimize the port timing