The way I understand it, four of you (one guy in the US and three guys in another country) got together and built a startup from scratch. I further assume that there were no discussions on equity split at any time.

Now when a new guy comes in, you are told for the first time that you have only 5% equity in the company.

I would suggest that before you do any further work in this startup, have a call with your US partner and speak to him about this - get everything clarified. If he does not speak to you directly about this, then you need to be very concerned.

It is very hard for HN to tell you how equity should be split, as no one here knows what each of you bring to the table. Typically, equity discussions should be carried out right before you start a partnership, so I urge you to have this discussion with him now.