Let me respond to each of your three concerns:
(a) Relative to your 4-year-old, her behavior appears to reflect more than the finicky, particular behavior not uncommon in such pre-schoolers. As I mentioned before, it would be wise to obtain an evaluation by a child mental health clinician. Check with your daughter's pediatrician about how you could secure such an evaluation. With such help, the nature of the behavior can be clarified: normal spectrum (probably not) vs. temperament/disposition vs. emotional disorder. The clinician will be able to offer you guidance about sensible ways to interact with your daughter. With older children who display OCD, medication can be employed, but this is not very likely with a child as young as your daughter.
(b) Relative to your 16-year-old, you are certainly correct in regarding her weight as a problem, even if she carries it well. At the very least, her weight poses a number of medical risks. She should be evaluated re: possible underlying medical problems (e.g., endocrine disorder, metabolic disorder) and possible psychological condition that may be resulting in compulsive overeating. The family history of obesity will be of particular interest when she is evaluated. She will also be able to obtain the guidance of a nutritioninst to help her with sensible eating, regardless of the ultimate diagnosis. Do you have health insurance? Check with your regular doctor for guidance about how to proceed. Your family's moderate means should not be an impediment to seeking help. In all likelihood, any treatment your daughter requires will be available.
(c) Relative to your friend's 6-year-old grandson, his eye twitching probably is of some concern. One of the possible side effects of Ritalin is the development of motor tics (involuntary muscle movements), usually of the facial muscles. If this is the case, the medication will likely be discontinued and the youngster can be started on a different stimulant medication to assist him with his hyperactivity (and any other symptoms of Attention-Deficit/Hyperactivity Disorder - e.g., inattention, impulsivity) he may display.