The SB29RDNC's produce an amount of 2nd order harmonics below 4khz, but that's typically not offensive. You probably want to cross at ~2,250-2,500hz to get the best out of them.

Here's how their THD looks at around 8 actual watts (which is pretty loud):

You can see 2nd order harmonics crosses the 1% threshold around 4khz. Odd order harmonics don't become much of an issue until around 1700khz at that power level. You'll be only putting out about 25% of the power with an LR4 crossover around 2.4-2.5khz at that frequency, which means your odd order and 2nd order harmonics will look a lot more like this at ~1600khz and further improve below:

So 2nd order harmonics will probably be in the -35db range (~1.77%) and odd order harmonics will be far below that with the most prominent being 3rd order at around 0.25% THD.

In other words, this should be a good performer and the 2nd order harmonics will likely just add a "warm" sound to the overall signature. I'd give between 2.25 - 2.5khz LR4 a try and see what matches your listening levels the best.