If you are using Arduino (or any other TTL device) to control 4 channel relays or more, I would suggest to use ULN2803 Line Driver instead of Transistors or FETs. You can use ULN2003 which is much more cheaper if you plan to use not more than 7 channel relays.
The ULN2803 Integrated Circuit (IC) is a "Eight-way Line Driver". It allows you to interface TTL signals (5v) with higher voltage/current (50V/500ma) loads.
As shown in figure 1 and figure 2, an Arduino is control 8 relays, one using ULN2803 line driver and the other one using transistors, both circuit are equivalent.
Notice that using a ULN2803 to drive 8 relays is much more simpler and cheaper compare to using 8 transistors. R1-R8 (current limit resistor) and D1-D8 (flywheel diode) can be eliminate when use with ULN2803 since the ULN2803 is already built-in with resistor & diode as shown in figure below.