You can safely call your friend's daughter 'lieverd' (sweetheart) or 'meisje' (little girl) as these are warm but not overly intimate pet names in Dutch. 'Schat' is indeed best reserved for close family, so your instinct to avoid it is correct. In Limburg, you might also hear 'sjoeke' or 'popke', which are local equivalents of 'sweetie' or 'poppet'.
For a nearly-four-year-old, simple and affectionate terms work well. 'Lieve meid' (sweet girl) is friendly and appropriate for a family friend. You can also use her name with a '-tje' suffix (e.g., 'Annetje' for Anna) to create a diminutive that feels caring. In Belgian Limburg, dialect options like 'sjoeke' (from 'schatje') or 'popke' (little doll) are common and endearing, but check with her mother first if you're unsure.
Avoid romantic pet names like 'schat' (treasure), 'liefje' (darling), or 'engel' (angel) as these imply a closer bond. Stick to terms that are parental but not exclusive: 'kleintje' (little one) or 'uk' (tiny tot) are neutral and widely used. If you want to be extra safe, just use her name with a friendly tone and a smile.
To decide, consider your relationship with the family. If you're close friends, 'lieverd' is fine; if you're more distant, 'meisje' or her name with '-tje' is best. You can also ask the mother casually: 'Wat vind je dat ik tegen haar mag zeggen?' (What do you think I can call her?). This shows respect and avoids awkwardness. In Limburg, dialect terms vary by village, so local input from the parent is valuable.
Ultimately, the child will respond to your tone and warmth more than the exact word. A cheerful 'Hoi, meisje!' or 'Dag lieverd' will feel natural and kind. If you're learning Dutch, practicing these phrases with her builds both language skills and rapport.