Same thing happened to my daughter until last year (she's now 5) but with Spanish instead of English, because I'm Spanish. When she was 4, she only spoke Japanese, but could understand me well because I have only spoken to her in Spanish. It had been 2 years without going to Spain, but last summer we could go at last and spent 5 weeks there on vacation. I can tell you, she had rarely spoken Spanish before that, and never more than a couple of minutes and not continuously, but since the moment the airplane landed in Spain, she automatically made a radical switch to Spanish and talked to me almost 100% in Spanish for the first time in her life. It was like a miracle and I really did not expect somthing like that. Those 5 weeks were very reinforcing because she could speak with family directly in Spanish for the first time and I also enrolled her in a normal Spanish school to be surrounded of Spanish kids every morning. After that trip, she speaks to me almost 100% in Spanish instead of 100% in Japanese as before the trip. Her level is a bit low compared to a native Spanish speaker of her age, but she's progressing every day since that. So my personal advice: try to travel to your home country and stay there as long as possible, no matter how expensive and troublesome it is. That change can happen at this age, and if it happens it will remain, but it will be very hard to happen later.