user6985: I like to play 4...e5 in the Open Sicilian in my games, usually facing something like this:
[StartPly "8"]

[fen ""]
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 e5 5. Nb3 Nc6

I do this to:

Chase the knight so that it doesn't block advancing my pieces.
Regain control of the center after 3...cxd4.
Gain a tempo.

Is there any disadvantage in doing this ?

user4588: After 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 e5 5.Nb3 Nc6, the position is probably fine for black.
However, after 4....e5, white seems to have a stronger reply: 5.Bb5+. According to the Game Database of ChessTempo, this is also the most popular move. Now, black has two options: 4....Bd7 or 4....Nd7. After both moves, white can put his knight on f5 and obtain a slightly better position.
Therefore, I would say that 4....e5 is a small inaccuracy. However, the ideas behind the move (occupying the center, gaining a tempo) are perfectly fine.
Maybe it is interesting to take a look at: 

the Sveshnikov variation: 1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e5
the Kalashnikov variation: 1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 e5
the Classical variation: 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 Nc6, and after 6.Be2, 6....e5
the Najdorf variation: 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6, and after 6.Be2, 6....e5



      [FEN ""]
      [StartPly "3"]

      1.e4 c5 2.Nf3 d6 (2...Nc6 3.d4 cxd4 4.Nxd4 Nf6 (4...e5) 5.Nc3 e5) 3.d4 cxd4 4.Nxd4 e5 (4...Nf6 5.Nc3 a6 (5...Nc6 6.Be2 e5) 6.Be2 e5) 5.Bb5+ (5.Nb3 Nc6) 5...Nd7 (5...Bd7 6.Bxd7+ Qxd7 7.Nf5) 6.Nf5



6985: <p>I like to play 4...e5 in the Open Sicilian in my games, usually facing something like this:</p>

<pre><code>[StartPly "8"]

[fen ""]
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 e5 5. Nb3 Nc6
</code></pre>

<p>I do this to:</p>

<ol>
<li>Chase the knight so that it doesn't block advancing my pieces.</li>
<li>Regain control of the center after 3...cxd4.</li>
<li>Gain a tempo.</li>
</ol>

<p>Is there any disadvantage in doing this ?</p>
 4588: <p>After 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 e5 5.Nb3 Nc6, the position is probably fine for black.</p>

<p>However, after 4....e5, white seems to have a stronger reply: 5.Bb5+. According to the <a href="http://chesstempo.com/game-database.html">Game Database of ChessTempo</a>, this is also the most popular move. Now, black has two options: 4....Bd7 or 4....Nd7. After both moves, white can put his knight on f5 and obtain a slightly better position.</p>

<p>Therefore, I would say that 4....e5 is a small inaccuracy. However, the <em>ideas</em> behind the move (occupying the center, gaining a tempo) are perfectly fine.</p>

<p>Maybe it is interesting to take a look at: </p>

<ul>
<li>the Sveshnikov variation: 1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e5</li>
<li>the Kalashnikov variation: 1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 e5</li>
<li>the Classical variation: 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 Nc6, and after 6.Be2, 6....e5</li>
<li>the Najdorf variation: 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6, and after 6.Be2, 6....e5</li>
</ul>

<pre>
<code>
      [FEN ""]
      [StartPly "3"]

      1.e4 c5 2.Nf3 d6 (2...Nc6 3.d4 cxd4 4.Nxd4 Nf6 (4...e5) 5.Nc3 e5) 3.d4 cxd4 4.Nxd4 e5 (4...Nf6 5.Nc3 a6 (5...Nc6 6.Be2 e5) 6.Be2 e5) 5.Bb5+ (5.Nb3 Nc6) 5...Nd7 (5...Bd7 6.Bxd7+ Qxd7 7.Nf5) 6.Nf5
</code>
</pre>
