1．H-1延期已经递交，先生用目前的 H-1申请 H-4入境，再单独递交 H-4 延期申请，是否可行?
Yes
2．如 H-4 延期可以单独递交，至少在现有 H-4 过期之前多长时间递交上去？需要提前 45 天递上去吗？
No. as long as the CIS receipt date is earlier than I-94 expiration date
3．如果 H-4 延期可以单独递交，H-1延期 receipt尚未接到，可否使用 H-1 延期时学校file的I-129 ， 代替 H-1 延期申请的 receipt？
I don't think so, and i don't think it's doable.
4．H-4延期单独递交，（在 H-1延期尚未批准的情况下，使用上述 H-1 延期 receipt或I-129），是否一定要找律师递交？可否自己file？
Yes you can file I-539 yourself. However, I would wait for H1B extension approval notice. There might be some people file H4 extension with H1B extension receipt. However, what if your H1B is not approved yet when they process the H4 case?
You either file H1/H4 extension together, or file H4 extension after H1 approval.
Yes
2．如 H-4 延期可以单独递交，至少在现有 H-4 过期之前多长时间递交上去？需要提前 45 天递上去吗？
No. as long as the CIS receipt date is earlier than I-94 expiration date
3．如果 H-4 延期可以单独递交，H-1延期 receipt尚未接到，可否使用 H-1 延期时学校file的I-129 ， 代替 H-1 延期申请的 receipt？
I don't think so, and i don't think it's doable.
4．H-4延期单独递交，（在 H-1延期尚未批准的情况下，使用上述 H-1 延期 receipt或I-129），是否一定要找律师递交？可否自己file？
Yes you can file I-539 yourself. However, I would wait for H1B extension approval notice. There might be some people file H4 extension with H1B extension receipt. However, what if your H1B is not approved yet when they process the H4 case?
You either file H1/H4 extension together, or file H4 extension after H1 approval.
