Your initial grant is not "$50K a year". It's for example, "200k over 4 years". And it's not actually in USD, it's in number of stocks. So if the GOOG stock is $1000, your grant would be "200 stocks vesting over 4 years".
Now when you get a promotion, you'll get a new set of stocks vesting over 4 years. If you get the promotion before the 4 years expire (usually the case) it will overlap with your initial grant.