Hi Tareq,
Alex has summed things up pretty well.
F4 has the advantages of giving you a wider TFOV for a given aperture and a shorter telescope. The disadvantages are that you will need to use a coma corrector and expensive high quality eyepieces, which better handle the steep light cone of an F4 telescope than cheaper eyepieces do. This of course does depend on how "picky" you are about the quality of the views.
As Alex has also pointed out, it is no harder to accurately collimate an F4 telescope than an F8 telescope. However, an F4 Telescope is far less tolerant of slight miss collimation than an F8 telescope, and when collimation moves off a bit the views deteriorate. With an F8 telescope the collimation can move off a bit and you don't notice it. As Alex also pointed out in cheaper telescopes they tend not to hold collimation all that well. This will mean that you may need to check and re collimate 2 or 3 times a night to maintain the best quality views.
The other thing which also changes with the faster F-ratio is the "depth of focus". This is the focus range that the image will stay in best focus. With an F4 telescope it is very shallow and you will find you continually need to re focus to keep the view sharp when viewing the moon and planets at higher powers. With slower F-ratios the depth of focus is much wider and the image will stay in sharp focus a lot longer without the need to continually re focus.
While the modern trend is to go to faster and faster telescopes because they are more portable and give a lower eyepiece height, my personal preference based on all relevant factors that are important to me, is that for telescopes over 12" aperture the "optimum" F-Ratio is about F4.5 and for telescopes under 12" the ideal F-ratio is between F5 and F5.5. All of my telescopes have been custom built and my 2 Newtonians over 12" are both F4.5; and my 10" is F5.3. With apertures over 22" you seriously need to consider faster telescopes between F3.5 and F4, as the eyepiece height and telescope size can get both dangerous and impractical. A friend of ours in the US (Larry Mitchell) has fallen from the ladder while using his 36"/F5 Obsession on more than one occasion and has broken several bones in the process. I have been up that 15 foot ladder and its a long way down and not really where you want to be on a cold windy night
Cheers
John B