You might try a little fresh ginger tea. It's an ancient remedy for asthma. Slice fresh ginger thin and steep in a ceramic cup for twenty minutes.
Ginger is a mild anti-inflammatory and far less harmfull than a steroid.
Also- pulmicort lowers your immune system some.
I also have a four year old with this problem. Like they said- it could be your lifestyle. My daughter sees a immunologist every 3 months. Has you child has a blood allergy test? If not...make that STEP 1. Pulmicort us to not work for us either because we had a cat. We got rid of the cat (she allergic) and after intense cleaning and 3 months...her asthma is almost gone! AKA under control
Another thought here .... have you ever had him tested for cystic fibrosis? The viral infections and lack of medications helping would be consistent with that diagnosis.
It would be worth asking for it (would be a simple sweat test that is non-invasive).
a child like your the only preventable thing is trying to to prevent him having this kind of viral infection.
by going to a nursery he will be prone to catch the flue from the children around.
try to keep him home, make sure to wear worm clothes when he goes outside.
keep an eye on what he eat, make sure of eating fruit and vegetables specially oranges to increase his body defense mechanism.
hope these help
Is it possible that you are living in a 'toxic' environment? Just thinking out of the box and I lived in a toxic house with hidden black mold with a toddler and we were both with either sinus issues or upper respiratory issues for 7yrs...til we moved a very large and heavy sofa and found the fuzzy, black mold growing on the baseboards from an AC unit in the window ( a huge AC).
We even took my son's tonsils and adenoids out after 12 antibiotics in 9 months.
Good luck.
The problem would be why your daughter is getting sick so often and increasing her immune system may help.
The best way would be to start heavy preventative medication as soon as there is a slight runny nose - pulmicort plus around the clock ventolin (or whatever SABA you are using).
Supplements that increase his immune system may also help (although talk to his doctor before starting any to ensure the proper dosage for a child). Vitamin D is a major one, especially if you live in north america and have poor sunshine access during the winter, NAC (N-acetyl cysteine) is also a supplement that helps thin the mucus and has been shown to reduce the symptoms of colds and flu (although I'm not sure if it is child-friendly). Vitamin C and B complex are also suppose to help.
Good sleep and food will make a big difference too for the ability of the body to fight off infections. Lots of fresh fruit and veggies and reduce the processed and sugar fill treats and meals because they are hard on the body to digest and process; which would take away energy that could be used by the immune system to fight off the infections.
Good luck and hopefully something will help!