Blue excitable red phosphor is the key component to improve the quality of lighting and display which is based on InGaN blue chips. Because of the potential in the area, Mn4+ red phosphors have recently got rising interests. However, most of them were found by trial and error. It remains very challenging to predict which kind of compound can stabilize Mn4+ and which wavelengths Mn4+ ions if they could survive in it will exhibit the excitation and emission at. Here, we first propose to use crystal field calculation on basis of exchange charge model to predict the energy levels of Mn4+ ion in germanate K2Ge4O9 since Mn4+ and Ge4+ are almost identical in size and charge, and the local field around Mn4+ will experience less distortion after substitution for Ge4+. The calculation shows the red emission peaking at 663 nm and the blue absorption of (4)A(2g) (F-4) -> T-4(2g) (F-4) in 450 similar to 470 nm, which matches better to blue chips than commercial phosphor 3.5MgO center dot 0.5MgF(2)center dot GeO2: Mn4+. This inspires the synthesis of Mn-doped K2Ge4O9, the optical properties of which confirm the existence of Mn4+ and consist with the prediction. Comparison between theoretical and experimental results implies that no obvious preference of Mn4+ substitution over two different types of octahedral germanium sites, Ge1 and Ge2. Consequent systematic explorations have been made on the effects of flux content, preparation temperature, Mn content, and holding time to find the ways to enhance the Mn4+ luminescence for promotion of practical application. The results reveal the optimal sample can be made under much mild optimal condition ( 850 degrees C for 4 h in air) with a quantum yield of > 30% upon blue excitation of 460 similar to 470 nm. As temperature rises from 8 to 573 K, zero photon line ( ZPL) emission redshifts along with gradual appearance of anti-Stokes side phonon bands due to the enhanced interaction of Mn4+ with host. This work demonstrates it possible to find Mn4+ red phosphors by guide of crystal field calculation.