What's the best solution for this funnel problem I'm having?

Hey all...

I have an auto-responder sequence set up in MailChimp. I make my "irresistible" offer on the 4th e-mail. Here's my problem...

I want to be able to push people who do not open e-mails through the funnel by re-marketing to them - automatically.

Example; I have a list of 690 subs where only 60% open the e-mail they opted in for.

Those 400 people get the second e-mail and maybe 200 of them will open that e-mail.

Those 200 get the third e-mail and maybe 100 will open that e-mail.

Those 100 get the fourth e-mail and maybe 50 will open that and a handful will buy...

So, I need to be able to automatically re-market to the 200 or so who do not open the first e-mail. I I want to make 3 attempts over 2 days to get them to open the first e-mail. Then, I want them to move on to step 2. I'd like to repeat the same process for all e-mails in the funnel.

Ex: If user does not open e-mail 1, re-attempt 3x. If user does not open e-mail 2, re-attempt 3x, and so on.

Essentially, I need an automated way to push my un-opens through the next step in the funnel by potentially re-visiting them with different subject lines, times of day, etc.

Here's where I hit the wall in MailChimp

1. I created the first e-mail in my autoresponder series.
2. I created 3 additional e-mails to target those who do not open the first e-mail.
3. Now I have 4 total e-mails. Initial auto-response + 3 attempts to target those who do not open the auto response.
4. I want the 5th e-mail to be able to cover ALL of those who open any of the first 4 e-mails. However, in mailchimp, I can only target those who opened the 4th e-mail. I can't set up e-mail #5 to target ALL of those who opened 1-4... :-(

I know this is a bit long winded but I've tried to be as detailed as possible... Any advice appreciated. Do I need to go to another service provider? Thanks!