first lets explain what crowding is, which is just your case.
Normally we have 32 permanent teeth, rooting in upper and lower jaws. The size of jaws and that of teeth match and thus teeth are positioned and aligned in the normal way.
If the size of jaws is too small, or if teeth are too big, especially too wide, the jaws cannot accomodate teeth and crowding happens.
Crowding has too basic signs,
1, malposition which means the whole tooth stays out of the line, either close to cheek or to tongue, but still straight up.
2, tipping or rotation.

To relieve crowding, either increase the size of jaws, or decrease the size of teeth, into which your case fell since four teeth have been removed.

Right now you have four empty SPACEs in your mouth after the extractions done. Next step is to align the tipping or rotated teeth that will take some of the available space. This step can be done by invisalign or braces. The space from extraction will become occupied partially by uprighted and derotated teeth and of course will become smaller but still there.
After this step is done or synchronously, all front teeth will be BODILY moved back and the malposistioned ones will be brought back into the line and the rest space will be completely occupied. And, with no space anymore, all teeth are straight up and in position.

It is very difficult if possible for invisalign to move teeth BODILY. It can only finish the last step by tipping teeth backward.
This is why you should go for braces and also the explanation to the part you 不是特别理解.

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