A tuning fork of frequency $512~\text{Hz}$ makes $4~\text{beats/s}$ with the vibrating string of a piano. The beat frequency decreases to $2~\text{beats/s}$ when the tension in the piano string is slightly increased. The frequency of the piano string before increasing the tension was:
1. $510~\text{Hz}$ 
2. $514~\text{Hz}$ 
3. $516~\text{Hz}$ 
4. $508~\text{Hz}$