If you take this tree [1] as canon, English and Czech are fifth cousins, once-removed. If you compared me to my fifth cousin, I probably would look nothing like him, but I might carry similar traits or certainly similar ancestry.

Although the article doesn't convey it very well (and I am skeptical of the finding itself anyway), I believe what they are trying to say is that these words all 'have not changed' in that they all have derived from the same eventual ancestor. That is, the word's pronunciation may have morphed, but the words were never abandoned in favor of new ones. An example here would be the English 'four' vs. the Spanish 'cuatro'. If you believe in the reconstruction of the proto-language Proto-Indo-European (and Wiktionary [2]) then these both derive from the word * kʷetwóres[3].

The Spanish one is a bit easier to understand.

   * kʷetwóres (PIE) -> quattuor (Latin) -> cuatro (Spanish)

Nothing too shocking there, I hope. Latin dropped the "es" off of the end and changed the first vowel a bit, then Spanish simplified the "uo" diphthong into "o" (perhaps through Vulgar Latin? [4]), and reversed or->ro, and of course changed spelling because it's a different language.

English, however, took a little bit odder of a path. Somewhere before Proto-Germanic, * kʷetwóres->* petwṓr, which was an irregular change (perhaps to mimic five, which was * pénkʷe->* pémpe [5][6]). Then, a very regular rule, called Grimm's Law [7], kicked in in the first millennium BC, which shifted p sounds toward ɸ sounds ("For English-speakers, it is easiest to think of the sound as an f-sound made only with the lips, instead of the upper teeth and lower lip."[8]). So * petwṓr->* fedwōr (Note the t got vocalized into a d as well) in Proto-Germanic. West Germanic then lost the hard d in the middle, to make * fewōr, which became Old English fēower, into Middle English fower, into Modern English four. So:

    * kʷetwóres->* petwṓr->* fedwōr->* fewōr->fēower->fower->four

Both of these are sets of incremental changes from a common ancestor. Individually, the small changes make some sense (I hope), but once you get several thousand years away from the root, words like "cuatro" and "four" seem like they are completely unrelated by sound, even though they're very related by ancestry.

I'm no historical linguist, but hopefully that helps explain why these things sound nothing alike. :)

[1]http://upload.wikimedia.org/wikipedia/commons/4/4f/IndoEurop...

[2]http://en.wiktionary.org/wiki/Appendix:Proto-Indo-European/k...

[3] The * indicates that the word is a reconstruction, not actually something attested to in found writings.

[4] http://en.wikipedia.org/wiki/Vulgar_Latin#Monophthongization

[5] http://en.wiktionary.org/wiki/Appendix:Proto-Germanic/fedw%C...

[6] http://en.wiktionary.org/wiki/Appendix:Proto-Germanic/fimf

[7] http://en.wikipedia.org/wiki/Grimm%27s_law (Yes, one of the Brothers Grimm!)

[8] http://en.wikipedia.org/wiki/Voiceless_bilabial_fricative