Hi, C.,
If he is doing the same things over and over then I get it, you can only look at trains for so long. These are the things that stimulate him right now, but the good thing is that at 4 yrs old his mind is a ginormous sponge and it is easy to find other things that stimulate him. With about 15 minutes in the evening and VERY little money you can help him get interested in other things.
color eggs (who says it's just for easter) all you need is eggs, food dye, water and vinegar. let him color them and mix the colors to see what blue and yellow make and so on. peel the eggs and let him eat one or help make egg salad with you. save the shells, let them dry and crush them and let him glue them to paper to make a collage.
make bean bags. just put beans in a children's size sock and tie off the end. you don't even need to sew. let him practice tossing them in a laundry basket.
draw or print of basic shapes or pictures. put rice paper (you can get it at Michael's or Hobby Lobby) over the picture and let him trace them out. Or let him use cookie cutters as stencils.
get pipe cleaners and let him make pipe cleaner sculptures.
help him make a drum from an empty oatmeal or coffee container that he can decorate with contact paper and markers or crayone. do maracas with beans, dixie cups and glue. he can make his own marching band.
put a fine layer of salt on a cookie sheet with his name written on the bottom in sharpie marker and let him trace his name, or do shapes. you can do the same with lotion.
let him play with shaving cream on the table. mix in a little bit of jello powder and it is colored shaving cream. do two different colors and let him mix them and see what color he comes up with.
tear different colored pages of tissue paper into small pieces and let him stick them to clear contact paper and make sun catchers to hang in the window.
Get window paint (I have even seen it at rite-aid)and let him draw art on the windows.
get a giant cardboard box and talk about what he wants it to be...a car, a fire truck, a puppet theater...and help him cut out the windows or draw on the decorations and then let him play pretend with it.
mix water with a couple of different boxes of jello to make edible finger paint and let him paint on a paper bag that you have cut open and made into large art paper.
get different clothes from the thrift store that will encourage dramatic play. Hats, scarves, shoes, boots, vests, etc.
Use froot loops cereal (the off brand of course) to sort colors and make patters and then make necklaces or bracelets.
The key is to find things HE hasn't done before. He will have a ball, he will be learning something new, and you will have fun watching him be engaged. Plus if he is occupied he will be practicing entertaining himself and won't need you to do it as much.
Hope this helps!
L.