This text is a development of a preprint of Ankit Gupta.

We present an approach for devising a deterministic polynomial time whitekbox identity testing (PIT) algorithm for depth- 4 circuits with bounded top fanin. This approach is similar to Kayal-Saraf approach for depth- 3 circuits. Kayal and Saraf based their algorithm on Sylvester-Gallai-type theorem about linear polynomials. We show how it is possible to generalize this approach to depth- 4 circuits. However we failed to implement this plan completely. We succeeded to construct a polynomial time deterministic algorithm for depth- 4 circuits with bounded top fanin and its correctness requires a hypothesis. Also we present a polynomial-time (unconditional) algorithm for some subclass of depth- 4 circuits with bounded top fanin.