Developmentally, four-year-olds shouldn't be expected to be able to talk about their feelings. They just aren't really good at identifying and/or reporting their inner emotions. It's just too abstract for them. Happy - Sad - Angry...that would be the extent I would expect from a typical 4-year-old. MAYBE "frustrated" IF this is something that a parent has taught consistently as an emotional label, but it's really too complex for a preschooler to be reliable about consistently.
The best you can expect is some basic labeling and/or the beginnings of appropriate emotional expression and self-control.
I would not try to engage in a "conversation" about it, asking him to report on how he feels. If he's not able to do it, he's just going to be increasingly uncomfortable because he can't do what you want.
1. I would use modeling - label your own emotions and appropriate emotional expressions. e.g., when you drop your purse and things spill, "Oh! What a mess - I have to clean this up and it will make me late for...I feel ____ (angry? upset?)" (I'm bad with examples, as I wrote that, I thought that maybe spilling a purse isn't something to get upset about but you can think of your own examples.)
2. As jessie119 suggests, I would label other's emotions - When you're watching TV or other children at the playground and an emotional thing happens, say, "That little boy fell down and is crying, I bet he feels sad."
3. After labeling the other's emotion, you can ask your son how he would feel.
4. When your son has an emotional event, you can label his emotion for him. An expansion of this is to label/ask at the same time. "It looks like you're struggling to get that toy to work, does that make you feel frustrated?" Obviously, you wouldn't want to do this unless your son is showing obvious signs of frustration. You don't want to suggest that working hard to get something to work should always lead to frustration.
Finally, a 4-year-old doesn't have the memory capacity to report what he did at school that day. A better approach is to ask him what his favorite thing he did that day was. Or, to ask if he did he have fun playing on the playground or who did he play with on the playground or did he use the slide on the playground. That way, you're providing the structure for the answer, instead of asking him to recall the information.
To help him develop his memory, when you tuck him in at night, review his routine for the day - first you got up, then you had breakfast, then mommy took you to preschool - you can decide on the level of detail, but you want to keep it relatively short. YOu can then ask him, "What did you do first at preschool?" or "Did you play with Susie at preschool today?" - and then continue to review the routine with him. Keep it short - about 1-2 minutes - and it's not a quiz, it's just the two of you review his life together. Also at that time, you can review the upcoming day, which is likely to be the same as what you just reviewed, but here's your chance to remind him of what may be different. "Tomorrow is Wednesday, and we always go to the library on Wednesday, so we have to remember to put your library books in the car when we go to preschool."