Namastream is not set up to do payment plans however, there is a work around that will require some manual tracking.
First, it's important to know that you set one price for a product. In other words, a product cannot have different payment options. If it's $400 fixed price, then that's the price and the student is not given the choice at checkout to pay in instalments.
However, you could create two products: one with a fixed price and one set up as a monthly recurring product to reflect what you want your instalments pricing to be.
Let's say you have a $400 course, that you would like to also offer as 4 x $100 instalments.
Here's how you would do it.
Create a product, Product A, with a fixed price of $400.
Create a second product, Product B, with recurring monthly amount of $100. This is a "dummy" product which is there to allow the students a second payment option. Recurring monthly will recur indefinitely, so the trick is to go into your payment processor after the fourth payment and stop all the recurring payments so the students don't overpay. This is the manual part.
Where do you put the content?
Put all your content into Product A (or whichever one you think you'll sell more of) so that everyone who purchases Product A will have access. But for those who purchased Product B, your "dummy" product, they won't have access to Product A because that's not what they purchased. To get all the Product B students access to Product A, you'll need to give all them a coupon (make it 100% discount) and ask them to purchase Product A with that coupon.