I feel like publishers would regard trilogies as a safer bet than a long-winded series.
But then again, publishers regard works from well-known authors as a safer bet, too. To paraphrase Brandon Sanderson (as talking of his latest series, The Stormlight Archives), once you start getting fame (and some sales below your belt) you will have more leeway, since your publisher will be more confident that you'll sell (or even, deliver).
As an unknown author, selling a 4+ series will be somewhat a difficult predicament, especially if those books are not self-contained. Selling a trilogy is already difficult (some readers will even look at the first entry of a new trilogy with distrust). Also, since your publisher doesn't know you, he doesn't know if you can pull it off.
Writing one good (publishable) book is hard enough, but delivering more than four? Without dropping the pace or the quality of your work?
Of course it can be done, but there will be plenty of reasons to be skeptic.
So, your best bet is to plan and sell your books as single, self-contained stories, with growing potential.
Let's imagine that you have already the first book written down.
Ideally, you don't want it to be just a "setting-up" book for later entries: it has to have its own story arc, its own character struggles, and its own resolution.
Once you have polished it enough, you could go to a publisher and say
"Hey, look at this. It's great for X reasons and you should totally fund it. Also I've already a sequel planned up if it goes well".
This is already more reasonable, from a publisher point of view.
You could try the same reasoning trilogy-wise:
"Hey, I've got this trilogy and it's great. You know, if it sells I've left space for more..."