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
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