请将下面这些合同语句翻译成中文，注意翻译时用语的准确和严密性。
1. This Contract can only be altered, amended or supplemented in accordance with documents signed and sealed by authorized representatives of both parties.____
2.合格投资者(QFII)必须遵守中国的法律法规和其他有关规定。____
3.有下列情形之一的，合同无效。____
4. If the seller fails to observe and comply with any of its obligations then the Buyer shall have the following rights.____