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: 1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e5
- the Kalashnikov: 1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 e5
- the Najdorf: 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.Bb5+ (5.Nb3 Nc6) 5...Nd7 (5...Bd7 6.Bxd7+ Qxd7 7.Nf5) 6.Nf5