According to Golem.de which claims to have got its paws on timescale information from  a SK hynix deep throat the 4GB will appear in the third quarter while the 8GB will show up in the fourth.

Both AMD Polaris and Nvidia Pascal will be using HBM2 memory. SK hynix is also producing 2GB stacks but it was not clear when they will be showing up. Samsung has already started mass production of 20nm 4GB HBM2. Samsung will also progress to 8GB HBM2 production at a point later this year.

The arrival of the SK hynix stacks will give Nvidia and AMD an alternative to Samsung and keep the price a bit lower.