It's very common for younger siblings to imitate and aspire to be like their older brothers or sisters and to want to play with their toys. The younger ones want to be part of the action they see going on around them.
Look for toys that have the types of activities your 18-month-old is interested in and are appropriate for his age. Many toys have a variety of things to do. They might even entice your older child to play along with your younger one. For example, the Little People® playsets, such as the Garage and the Busy City have lots of action, sounds, figures and vehicles.
Also, observe what attracts your younger son to his older brother's toys. Based on those interests, look for toys they could play with together. However, since there is a gap between their developmental levels and play needs, it is important that when they play together they have opportunities to play at their own pace. For example, your younger son will probably be interested in all of the action, putting things in and out and making things go up, down, and around. Your older son, however, will be more likely to make up stories, conversations, sound effects, and more elaborate play scenarios. With your guidance and example, their playtime together can be very enriching for both of them, especially when they learn to respect each other's differences.
For the next couple of years, your 4-year-old needs to be careful about keeping toys with small parts away from his younger brother. He needs to understand that even though his younger brother finds those toys interesting, they are not appropriate for him yet.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.