My 21 month old has had hfmd 4 times within 6 months with each time being as severe as the last. specifically; ulcers in the mouth. is this normal?

Not HFMD: This is not likely hand/foot/mouth disease. HFMD is a one-time illness to which you develop immunity and don't get it again. There are other viral infections that cause recurrent mouth sores, most commonly a herpes infection. May want to culture oral lesions next time they occur and see if indeed a herpes infection, but can tell you it is NOT recurrent HFMD.

YES: There are 2 -3 strains of enterovirus that I am aware of that can produce the HFM syndrome.Each confirm immunity afterwards. If the child is having a recurrent pattern of mouth ulcers and rash I would look to another explanation for the pattern. Are blood counts checked during this event ?

Unlikely to be HFMD: Recurrent Hand Foot Mouth Disease (HFMD) is rare -and four recurrences seems even more unusual. I wonder if another disease could be at work and this should be investigated. Other diseases can cause oral ulcers including inflammatory and immunologic diseases - so I would want an ID specialist and/or a rheumatologist to weigh in on your child's case. I think you need a definitive diagnosis here.