I would never put resistors in series because you are just robbing your speakers and turning half the current into heat.
Your reciever will most likely be fine if you listen at moderate levels, or you add an external 2 channel amp for just those speakers.
If you can find an impedance plot or have the know how to do it yourself (and you can do it yourself, it just takes a bit of doing), you might be suprised to know that those 4 ohm speakers are only 4 ohm for maybe 100-200hz, and they rise to 8 ohms for most of the bandwidth, and the 4ohm rating is only a minimum, manufacturers do the rating of resistance in all sorts of ways.
If you turn your speakers down 3 db, you will be needing the same current as if you were using an 8 ohm load, so just consider that, and keep your reciever cooled, and if you really want to be safe you could calculate how much amperage your reciever can put out safely and put a fast blow fuse on your speakers so you know when you've hit that level and not to go that much.
If you must use resistors (and I say this is a very bad idea), do not buy from radioshack, but buy from partsexpress, etc, and look for non-inductive mills resistors, so you don't degrade the audio signal as much, but there really is no reason to do this, because it does the same thing as turning the speaker down 3db, in essence.
If you have any questions about measuring impedance I can point you to some resources, and its a fairly cheap operation just takes some know how, and doing and time.