Tell us what version of the OS you are currently running on this iPhone 4 that is locked to Softbank. If it is running iOS 5 or 6, there isn't a known unlock.
However, just as here in Canada where I live, there are probably two ways around this:
1. Pay a specialist to do a factory unlock for you. If I did this here I would expect to pay about $100 or so.
2. Ask Softbank to unlock it for you. Many carriers (not all, but most in North America) will unlock an iPhone for a smaller fee. Usually they will require that you have completed your contract, but some companies only require that you have been a customer for 90 days (sometimes a bit longer). I do not know what Softbank's unlocking policy is but it can't hurt to visit a company-owned outlet (not a reseller) and ask.
I hope that helps.