Four overs is a fifth of a T20 innings, and it is fair to expect a team to field at least five bowlers in their line-up. Bowlers have managed to pick up four- and five-wicket hauls quite regularly in T20s, proving that four overs is sufficient for them to make an impact.
A four-over burst doesn't constitute a spell. It doesn't allow strike bowlers to fully express themselves and influence results, especially in low-scoring games. There is no limit on how many balls a batsman can face, and it's only fair that bowlers aren't saddled with unnatural restrictions either.
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