I don't think there is any solid (haha) evidence on whether 4 vs 6 months will reduce or increase the risk of allergy, asthma, eczema etc. Many organizations still say 6 mo is best, while the American Academy of Allergy, Asthma and Immunology has now changed its stance and says 4 mo is better, and Australia has just changed back to 4 mo too.
The only consistent evidence is that gastroenteritis is rarer if you wait longer until 6 mo, while babies' micronutrient status (iron, zinc etc.) tends to be a bit better if you start earlier than 6 mo.
I started with iron rich foods at 4.5 mo but just little tastes and paying careful attention to hygiene. No allergies or asthma here, so that's my anecdata.
Do you personally feel your child is starting to need something more than milk? As a general rule, I hate the cliched advice "Trust your instincts" but when the data is so conflicting it's difficult to know what else to do.
The current guidance is 6 months, once they're showing signs of readiness... I'm waiting until 24 weeks at least, I believe it's due to the digestive system not being ready until then causing allergies and intolerances to be more likely - I'm no expert though.