I'm the mom of 3 kids and all were different with dressing themselves. I'm trying to think what they were each doing around age 4...
DD1 was undressing herself before DD2 was born (so under 16m) and dressing herself by age 2. I'd be busy with the baby and she'd go off and fuss with her clothing and get it figured out by the time I could give her attention.
DD2 didn't need to dress herself so much, as when she was a toddler/preschooler she was in daycare and often I just needed to rush out the door so I'd dress her, while her sister at the same age could play with her clothes all day and figured it out. Then a little bit older DD1 would help DD2 with clothes even if I didn't. I'm not sure at what age she was putting on what items, but I'm pretty sure she was dressing herself by the time DS was born (age 5.5) I honestly don't remember what she was doing in preschool, other than helping my mom velcro her shoes (as my mom was recovering from a nasty car accident that shattered her hip.)
DS still doesn't like dressing himself, but he'll do it. I'm slowly weaning him off of needing company while dressing! He still needs training wheels on his bike and just learned to swim last summer at age 5. At age 4 I think he was able to dress himself, though he didn't like to. With socks, I discovered that a certain brand and style he could do alone (after lots of explaining how to use 2 hands to do it) but other styles of socks he couldn't manage- at the time I bought him more socks in the larger size (in his preferred style) to encourage independence; now we still have most of them (some have worn out but all still fit.)
Kids grow and develop skills at different rates. It's especially common for a child who's advanced in one area to be a bit "behind" on another- total development is "average" but some kids just develop more fully in one area than another, and others develop more evenly.