Here are some four-letter manly names, alternative to Coco, keeping in mind that finding perfect alternatives with the same sound and feel is difficult:

Similar Sound/Feel (but not exact matches):

• Boar: Strong, rugged, evokes a sense of power.
• Colt: Suggests youth, energy, and strength.
• Dale: Short, simple, and somewhat rugged.
• Dean: Classic, strong, and somewhat sophisticated.
• Drew: Short, friendly, yet still masculine.
• Jake: Common, but short and strong.
• Kent: Classic, strong, and slightly sophisticated.
• Leif: Unique, Scandinavian feel, strong and nature-related.
• Nero: Strong, historical, and slightly dark.
• Rex: Short for "Rex," meaning king, powerful and regal.

Completely Different, but Manly:

• Ares: Greek god of war.
• Chad: Common, but short and masculine.
• Dirk: Strong, slightly old-fashioned.
• Finn: Short, strong, and slightly mysterious.
• Hawk: Strong, powerful, and evocative of nature.
• Jett: Modern, strong, and slightly edgy.
• Leon: Shortened version of Leonidas, strong and powerful.
• Luke: Classic, strong, and friendly.
• Mark: Common, but short and masculine.
• Thor: Powerful Norse god.

This list offers a variety of options, ranging from similar sounds to completely different names, all aiming for a masculine feel. The best choice will depend on the specific context and desired nuance.