What model speakers are they? To say they are 4 ohm speakers is simply not enough information to make any solid recommendations on. The most important factors are the phase angle where the minimum impedance occurs and what frequency the maximum phase angle occurs. Without this information, buying an external amp may be simply over kill and a waste of money.
Take for example my speakers which are nominally rated for 6 ohms but live around teh 4 ohm mark in the upper bass lower midrange. My receiver has no problem driving them to 90 db seated 12' away without strain or fuss.
Here are their impedance/phase angle curves.
The phase angle stays below 45 degrees in the low bass region which is where the receiver would have the most problem driving them. Adding a sub and crossing over at 80 Hz lessens teh demand on the receiver considerably.