At 4 months a baby doesn't need the extra iron (or so HV told me!) so I wouldn't risk the constipation. Really they are still getting all the need from milk at this age. To help with constipation, or avoid it starting, you could try pear and baby rice. Is worked for us! Introduce one first, then the other around 3 days later just to check there is no adverse reaction.
Good luck, solid food is loads of fun!