I will not focus on the conferences vs. journals angle here, as this has been answered by other people and I really think this is not the core of your question. I will assume you meant to ask:

"Publishing 4 or 5 medium/low-quality venues or publishing only one paper in a top venue. What is better for PhD admission?"

Firstly, I am not entirely sure in which timespan you plan to produce all these materials. In my subfield of Computer Science, writing 4-5 B-level conference papers takes most PhD students at least 2 years. Writing 1 top paper requires a very good idea, solid research skills, and typically at least one half-year of full-time research (often significantly more). Doing all of that as a (presumably) inexperienced undergrad or master student besides course work seems very ambitious. From my personal experience, a very good master student will publish 1 or 2 good papers during his master's. That's about the best I have personally seen among my students.

Now, the simple answer to your (implied) above question is that both are likely ok. Both, one top paper or 4-5 reasonable papers, are likely to get you into any of the northern european school in principle. However, note that admission to european schools is often not like in the US (see also here), meaning that it is well possible that you still need to find a professor to take you on, which may depend more on her/his available fundings than your CV.

However, when you said "4 or 5 medium/low-quality" papers, make sure that they are not too low-quality. There is a threshold from which a badly conceived paper can actually hurt your chances in some groups. It is hard to give a hard-and-fast rule here, but in the dark I would avoid any predatory journals as well as any conference that does not appear on any of the international rankings (e.g., CORE). If you have an advisor or mentor from the field, he will be able to help you with selecting reasonable venues.

I will not focus on the conferences vs. journals angle here, as this has been answered by other people and I really think this is not the core of your question. I will assume you meant to ask:

"Publishing 4 or 5 medium/low-quality venues or publishing only one paper in a top venue. What is better for PhD admission?"

Firstly, I am not entirely sure in which timespan you plan to produce all these materials. In my subfield of Computer Science, writing 4-5 B-level conference papers takes most PhD students at least 2 years. Writing 1 top paper requires a very good idea, solid research skills, and typically at least one half-year of full-time research (often significantly more). Doing all of that as a (presumably) inexperienced undergrad or master student besides course work seems very ambitious. From my personal experience, a very good master student will publish 1 or 2 good papers during his master's. That's about the best I have personally seen among my students.

Now, the simple answer to your (implied) above question is that both are likely ok. Both, one top paper or 4-5 reasonable papers, are likely to get you into any of the northern european school in principle. However, note that admission to european schools is often not like in the US (see also here), meaning that it is well possible that you still need to find a professor to take you on, which may depend more on her/his available fundings than your CV.

However, when you said "4 or 5 medium/low-quality" papers, make sure that they are not too low-quality. There is a threshold from which a badly conceived paper can actually hurt your chances in some groups. It is hard to give a hard-and-fast rule here, but in the dark I would avoid any predatory journals as well as any conference that does not appear on any of the international rankings (e.g., CORE). If you have an advisor or mentor from the field, he will be able to help you with selecting reasonable venues.