Hello Alisson,
I'm one of Marketo's deliverability experts. Every 3-4 weeks is a pretty good frequency. Here are some key points to consider when making a decision like this:
*What is your sales cycle? Shorter sales cycles do better with faster messaging, while longer sales cycles do better with slower messaging.
*Other than this nurture track, will you be sending your leads many other messages? You want to make sure that event invites and special promotions don't combine with a nurture track to overwhelm your recipients.
*Realistically, do you have enough good content and enough time/personnel to write good content to mail at the frequency you're considering?
Most companies do best sending 1-2 emails per month, so that's a pretty safe bet.