Viber messages

Hello, I have a Sony Xperia Aqua Dual smartphone. This is about the Viber messaging app. I would like to know what the small ticks mean that appear when my messages are sent. I have one grey tick for sent... and I don't know what two grey ticks mean... and two blue ticks... especially the two grey ticks.
Thank you very much...

Before, it showed read or seen, now I have these small ticks instead.