The government's provides interactive "Proyecto Biosfera" for 4th grade. This is not a PDF, but an interactive website that covers 100% of the Natural Science curriculum with printable worksheets.
If you are looking for specific topics to study, the Santillana Natural Science 4 program typically includes: Natural Science 4 Primaria Santillana Pdf
If you simply cannot afford the Santillana book, do not resort to piracy. Spain’s Ministry of Education and regional governments (Junta de Andalucía, Comunidad de Madrid, Generalitat de Catalunya) offer free, high-quality PDF textbooks. Comunidad de Madrid