No you don't need to post proof of participation anywhere. Just meet all the requirements, complete the form and everything is good

I did everything, completed the form one minute after the start, obviously did some mistake since I didn't recieve any coins.
Am I allowed to apply for the next airdrop?
Yes you can 
The most common mistake was people not pasting the correct link for their Bitcointalk profile. Make sure you're link has numbers in it. You can find it by clicking your profile name in a thread like this, or by going to profile and clicking show all posts and copying that url.
Thanks. Will be more careful this time.

So, this happened. Today I found my MM address in the spreadsheet for AD3 (not AD4 in which I participated). Obviously I wasn't aware of this and I applied for the 5th airdrop.

Will I be excluded completely or just from the last one?

As we had spare places in Round 3, the first applicants for Round 4 got moved up to Round 3 to fill the spaces, so you'll get more PLEX as a result. As you applied for Round 5, your application would have been caught as a duplicate and therefore not processed. You'll still be in Round 3 so you don't have to be worry
