What app would you recommend for a 4 year old who can’t retain letters? The mom has dyslexia, so we think the 4 year old might as well.

If you are asking about remembering letter names, any games that would have repeated practice, including those that work on sounds and handwriting.
We have a lot of apps here. And you can filter them by ‘preschool’ and ‘letters and sounds,’ as well as by operating system.

Playing memory/concentration is a fun way to practice. You could easily begin with familiar letters that are in the child’s name, pet’s name, etc. I recommend beginning with a few letters and then add 1 letter at a time. I’d try to steer clear, initially, of those letters that are easily confused—b, d, p, & q—unless they are in familiar names or words (such as favorite toys). The child could help write the letters (even copy them) on 2 sets of cards to make a matching game. You could use them for Go Fish as well.

Also pairing the letter with a familiar word/picture to help remember it—m for mom. You could download pictures or draw them. Anything to make it fun.