plz tell me how to do six grid refrence four grid ref

and about everything related to topography

The grid is a set of lines which are used to find the exact location of places on a map. The horizontal lines in a grid are known as Northings, whereas the lines which move vertically are known as Eastings. It is important to note that the Eastings are given first and then the Northings

The four grid reference is used to find out the location which lies in a given figure (usually a square shaped box).  In four grid reference, the first two figures give us the Eastings whereas the last two give us the Northings. Four grid reference are mainly used for locating large lakes, settlements, embankments etc.

In six grid reference, the first three are the Eastings and the last three are Northings. Spot height, post offices and police stations can be located with the help of six digit grid reference.

The following will help you to mark the four and six grid references

In the given figure, find

1. the four grid reference of P, Q, R
2. the six figure grid reference of B, C and D
3. 

Four figure grid reference of the following points are:

P: 4465                   Q: 4162                        R: 4064

Six figure grid reference of the following points are:

 B: 431604                C: 458638                       D: 448612