Repeated blissful experiences in meditation When I first started meditating and after a while, I had a period where I experienced extremely wonderful sensations in my body. 

It culminated one day when I was meditating, laying on my bed: I felt like my body has vanished, time stopped and extreme pleasure arose in my body. It was like I was floating in a substance full of pleasure to my mind. I didn't feel my body, it was like my body vanished, I had no thoughts, but was fully aware. I was in this state for 4 hours, and if it wasn't for somebody calling me, I would be even longer. When I emerged from this state it felt like 10 minutes passed. This experience was the most wonderful, joyful, pleasant experience I had ever experienced in my life.

The next time I went into meditation, I had "a kind off" similar experience as described above, but it was different: It was less "mentally powerful". I didn't search for it, nor wanted to have it - I just went into meditation as in my last meditation session when the pleasurable experience arose, but this time the experience had a lesser impact on my mind and body: it didn't give that much pleasure to my body and mind as the first time. It was like I got accustomed to it, and my body and mind were more calm, thus I never noticed that much pleasure and joy as the previous time.

At one moment in the meditation I made an attempt to search for the pleasure I experienced the first time, but I quickly noticed that it just makes my mind wander away from meditation and fills it with "want". So, I stopped "wanting" and just let it go.

The third time I went into meditation, the described experience subsided completely and only calmness, emptiness, no thoughts, neither pleasure nor non pleasure, remained. 

I was not bothered forcing the described experience, because I knew that the experience came by not "wanting" it to came. I noticed that if I wanted it, the calmness, no thoughts, neither pleasure nor non pleasure, would subside and my mind would be permeated with "want". Thus, I abandoned "wanting" completely and just let it go and never ever tried to search for it.

I read about the 8 jhanas and during my meditation sessions I experienced "infinite space", "infinite consciousness", "emptiness", "neither perception nor non perception". I noticed this pattern: when the experience arose for the first time, it was the most powerful experience ever. It was like eating the most delicious cake for the first time in my life! The second time, it was less powerful. And so on, until it subsided and the experience "just was". Nonetheless, these experiences were never that much powerful as the first experience I described at the beginning of this question. Is this normal?

Now when I meditate, my mind is calm, one pointed, with no thoughts arising, and neither pleasure nor non pleasure. I can clearly see how and why everything is arising and ceasing in my mind. But I choose not to observe that anymore, I just let go of everything and the arising and ceasing in my mind stops. Everything stops, but I'm fully aware, and the experience is not that powerful as the experiences I described, when they arose for the first time. The experience that I have now during meditation "neither is nor it isn't". I could turn my meditation in an experience that "is", but i choose not to, because then "I'm not". It's hard to explain. From a Buddhist perspective, is this ceasing and letting go of extremely blissful experiences during meditation normal? Or does meditation lead to more and more powerful experiences?

Since that first time, when the described pleasurable experiences arose, they were never again as powerful as the first time they arose. It's like they subsided and only the core remained. From a Buddhist perspective, is this normal? Or should the described experiences always arise and be as powerful as the first time they arise?
From the theravada perspective, you ought to master each jhana. That is, you should have reasonable control of it, enter when you make a resolution to enter, keep it stable and leave when you make a resolution to leave. So you should be able to replicate it. If you identify important factors that are weak, these are factors that need further development. Weak jhana factors mean one does not fully understand their conditions, what obstructs them, how to cultivate them, control and keep them stable.

This is roughly the full jhana/samadhi meditation curriculum, though it can be argued that full mastering of jhanas are optional when pursuing the goal, in favor of mindfulness/sati meditation.

I believe Pa Auk Sayadaw considers a student to have mastered a jhana if he or she is able to enter at will and stay for 3 hours in it (period that testify the meditator skills in stabilizing jhana). This is also what he recommends before moving to the next jhana, which can be impossible to access without a minimum control of the previous jhana (e.g. as the factors to be abandoned might be too strong and unfamiliar to be properly controlled)