A class Valentine's exchange assumes a level of handwriting most four-year-olds do not have. The workaround is not writing the cards for them.
A name stamp, or a printed sticker with their name, on a card they otherwise decorated themselves, keeps the card genuinely theirs without asking for handwriting they do not yet have.
Colouring, stickers, a stamped heart — that is a complete card at four. A blank line for "to" and "from" that an adult fills in around it is a reasonable division of labour, not a shortcut that cheapens it.
Print or copy one design twenty times and let your child decorate the whole batch in one sitting, rather than starting a new card from nothing for each classmate. The exchange only needs the cards to look handmade, not to be twenty separate creative acts.
If they want to and it is not a fight, yes — even a wobbly attempt is worth more to a parent than a stamp. If it becomes a battle over twenty cards, use the stamp and let them decorate instead.
A name and a small drawing is a complete Valentine at this age. Nobody in the class is reading the sentiment.
Part of our guide to Holiday hosting with kids.