Opening at the 4 level  

Vulnerability

Always consider the vulnerability before a preemptive bid. Ideally you want to be not-vulnerable and the opponents to be vulnerable

What you need to open at the four Level

4♠ and , 4♥ are preemptive bids, this bid does not apply to minor suits because the 4 level is not game so the risks are greater than the rewards.
Preemptive bids prevent opponents from finding the best contract. when you make this bid you are promising a strong 8-card suit and less than 13 TP's. you must also have a good suit (a suit with at least two of top three honors).

Point Count Calculation

1. If you have Two of top three honors you can bid with a minimum of 5 HCP.
2. When calculating the 'less than 13 points' you include distribution points for length, so, an 8-card suit gives you 4 long suit points calculate 12 points  minus 4 long suit distribution points and you need less than 8 HCP to open at the 4 level.
3. The playable range for preemptive four opening bid is 5-8 HCP