We are coming to China in September. Our travel agent have as staying in Xian for 4 days and Beijing for 3 days.

Both of the cities are worth 3-4 days' visit. But as Beijing is the capital city of China and boasts the Great Wall, travelers usually spend a little bit longer there.

I recommend you arrange your internal travel by turning to Travel China Guide. They would help you book the train tickets with a little service fee. All is worth! If needed, here is their contact: bookingtravelchinaguide.com

Dear Simon,

Thanks for your attention to TravelChinaGuide.com! For visitors who first come to China, we would recommend relatively longer stay in Beijing. Apart from tailor-made tours, we can assist you with internal travel arrangement. Please feel free to send us an email at bookingtravelchinaguide.com and we would arrange your train travels.
Looking forward to your message.

Kindest regards,
TravelChinaGuide