Breaking number codes

You are trying to break a four digit numeric password. You guess those four digits , if you hit the bull's eye , you are done. Else if your answer is partially correct, then you have to guess the rest of the digits in the next attempt.Can you break within three attempts?

Prove that you need at most ten attempts.

Which is the best* algorithm when you have to do the job in three attempts or <10 attempts in general ? For example we may consider the fact : probability that two same digits are present next to each other =0.216

*suppose there are two algorithms A1 and A2 which can break subsets of all four digit numbers S1 and S2.If S1 belongs to S2 then, A2 is better than A1.

A problem from cryptography[S]

This field deals with converting plain text into cipher text ( un understandable ) using a mathematical construct to keep the information secure.A monoalplabetic substitution of +3 applied on the plain text "ram" gives cipher text "udp".Here 'mono alphabetic substitution' is the algorithm and '+3' is the key.
by knowing the algorithm but not the key can we retrieve plain text from cipher text ? [S]