techelit - tziztit

Is it permissible to use techelet on only one of my four tzitzit knots? Or, do all 4 have to have it if I use it? Thanks!

I was told by Rav Dov Lior shlit"a , the Rabbi of Kiryat Arba, that if there is Tchelet on only one of the knots, you should not make a Bracha on the Tzizit. Furthermore, Rav Lior said that since Tchelet today is available there is no justification for putting Tchelet on only one corner of the Tzizit.