Patterns are something kids start to learn in kindergarten in my area. That is after the age of 4. With your son's developmental issues, that might be ambitious to expect him to do a pattern on his own. I'd instead work on his just being able to bead which takes fine motor coordination. I'd put all the yellow beads together, all the red ones, blue ones, etc. Then say, pick a yellow one (if he has learned his colors) and direct him without expecting him to get the pattern himself.
Again, writing at 4 is often a challenge for kids with NO developmental issues. I think you may have unrealistic expectations for a 4 year old in general let alone one with MR.
My son is quite bright but fine motor issues were difficult for him. We began early writing with him AT FOUR. We worked with an occupational therapist and I'd say what you are trying to work on is not age appropriate fo any 4 year old.
There is a program that I do like called "handwriting without tears' that will start with basic letter formation, writing and move on to printing, then to cursive. You can get it online.
good luck ---- does he see an interventionist at all?
Here are some links for 4 year olds. Consider that with developmental delays you may need to back up and look at 3 year old skills as well. I provided these links so you can see what he should be doing if he had no delays. The items you mentioned are probably a bit over his age level w/o delays. Take it slowly, you want him to enjoy learning and expecting him to do things above his ability may backfire and he may become resistant to learning. What are his delays??
We are also using "handwriting without tears" for our daughter. She is 5 years old and has autism. She still does not understand patterns, but she can string beads. So, I wouldn't worry about that for now with your little one. Focus on the basics. Introduce her to simple milestones and work from there. It may be beneficial to seek out occupational therapy to address these concerns. It's not something you want to do on your own.