How to rotate delivery servers in the right way? I am stuck



I have changed the settings for delivery servers in backend - settings - cron.
I have 10 delivery servers and they should rotate. I set to change at 4, but the change took no effect.
The reason I want to change after every 4 emails sent is, I have some 3rd party smtp, they have extremely good deliverability for gmail, but they can only send 4 emails per hour.

What's happening is, mw tries to send 50 emails through this smtp. So after 4 emails, sending stops, and mw waits till the hour is over and next 4 are sendt.

How can I tell mw to send emails trough other delivery servers in the meantime?
I already set probability of these servers to only 20% but still this happens.

What is even more weird is that mw is sending other emails through other sending servers and there seems to change after only 1 or 2 emails sent.

Like, someone signes up for the list and receives welcome email. This Welcome email, only one - is sent out through fast smtp. Then 2 people sign up six hours later, and welcome emails are sent through a different fast smtp.
At the same time, the regular campaign to my main list is going through slow smtp at 4 emails per hour, and does not change delivery server.

Is there anything I can do about it?

I mean except segmenting my list and sending to gmail segment through slow servers, and rest segment through fast servers?

I am still on mw 1.5.1 btw ...
Thxs both. I set to 10s
I also checked for settings in groups, they were correct.
Maybe the 10s did it. Now working.