my kittens is 21 days old when can I start to feed them food

I;m wondering what's the mean of 4th week .. is it at start of  day 22 or day 29

because my kittens still toothless can't move well and I introduce food to them they didn't care it even...

also the litter box "wall" still to high to "climbs"....

they still really small.. any ideas

There is no hard and fast rule, as some kittens are more interested in solid food earlier than others.  If you have the mama cat, they will eventually start getting into her wet food (and she should be getting wet food at least while nursing!)   Yours are still too young t eat solid food even if they were orphans as they cannot digest it yet.  They should be on others milk if you have a mama or formula if you do not until they are at least 5 weeks old, and then you add a little baby food chicken to the formula.  If they have a mama, just let nature take its course.  Also, until they are eating solid food, there is no need for a litter box.  If they are orphans, you will be stimulating them to go by wiping with a cotton ball dipped in warm water to simulate the mama cats tongue.  If you have the mama, she will be doing all the toileting herself until they are eating solid food and then they will start having more solid poop. 

Get back in touch when that time comes and we will instruct you about litter boxes.  They should not have clumping litter at first because almost all kittens eat litter when they first discover it.  Plain sterile sandbox sand is the best until they stop eating the litter.  Also low cardboard boxes like cat food cans come in are the best for kittens to learn in.  Put them in all corners when the time comes because kittens naturally seek out corners so it is good training that they use the sand and not the floor or carpet.