The fact that silicon has four valence electron doesn't mean it has a similar chemistry. The main problem would arise when you start considering higher order bonding. Carbon readily makes double and triple bonds, allowing for a very rich chemistry. Silicon... not so much. Sure, you can find molecules such as Si2H4 and Si2H2. But the bonding scheme is very different. If we take C2H4 and tear the double bond apart. We end up with two fragments CH2, sp2 hybridized, with one electron in the free sp2 and one electron in the pz. This is the electronic ground state of the fragment. Let's go to silicon and do the same. For simplicity, we'll assume the fragment is also in a sp2 hybridization. What you will obtain is 2 electrons paired in the free sp orbital. So what do we need to make a carbon style double bond? First we need to unpair the two electrons and place them in both orbitals and bring two fragments together. Problem is, the cost of unpairing the electrons in two fragments is higher than the energy released by forming the bond. What really happens is that the two filled orbitals will overlap with the two empty orbitals and form what is known as dipolar or dative covalent bonds. So yes, silicon is just below carbon in the periodic table, but a chemistry based on silicon would definitely lack some of the versatility that carbon based chemistry can bring.
