I am able to successfully blend my voice: by following the sound of a baby cry I could bring my head voice to chest voice pitch, at least until f4, and I am able to sing that way, but I found that my voice volume is not as high as it used to be when I use only my chest voice. So, what do I do to increase my amplitude without pushing my chest voice but still following the blended voice?
It depends on the note that you're switching. If F4 is a high note for you, it's really difficult to blend to that note in a smooth way. For me, for example, if I'm going from high to low, and I want to follow that blended voice, I have to "switch" in B3-C4, but if I'm going from low to high, I can switch on D4.
It seems to me that you're working on what some people call Mixed Voice and in order to do this mixed voice I believe you're always sacrificing some volume. You might gain some body and some harmonics if you open your jaw more.