Hello, thanks for asking the questions. Fundamentals are the key: Remember the basic function of all switches: Forward, Filter or Flood. Each port us an RJ45 jack using twisted pair cabling (8 conductors and 4 pairs)--1 pair is used for tx, 1 pair is used for rx in a FAST ETHERNET connection.
In your first question: PC1 is connected in port 1 and PC4 is connected in port 4 what will happen when PC1 send data to PC4 and PC4 send data to PC1 at the same time?
PC1 will transmit to PC4 on the sending pair, while receive from PC4 on the receiving pair.
PC4 will transmit to PC1 on the sending pair, while receive from PC1 on the receiving pair.
this can happen simultaneously because of the separate wires.
In your second question: PC1 is connected in port 1, PC3 is connected to port 3 and PC4 is connected in port 4 what will happen when PC1 send data to PC4 and PC4 send data to PC3 at the same time?
Even though the Fast Ethernet defaults to full duplex, in this instance, it won't. PC1 and PC4 will transmit simultaneously but PC3 port will buffer the transmissions to handle it. Then PC3 will receive what will look like a single stream of data. Now, the exception to this is that all switches do it differently in some manner depending on whether the switch is a cut through switch, store and forward etc... the little unmanaged switches will all handle this a little differently.