Sounds like your first three have similar age gaps to mine! (16 months, then 2 years, then 19 months). For preschool we did Twenty-Six Letters to Heaven (Sarah Park) and overall I liked it. Obviously they don’t need to be doing academics yet but it made me be more intentional about doing stuff for my oldest instead of making him fend for himself as much as possible. He also really enjoyed it I think, and it was easy to lump the 3 yo in with whatever she felt like doing.