How secure is a 4 digit seed? When I Use a 4 digit random number(e.g. 4468) as seed for a (manual) Stream cipher and straddle it to the length of the plaintext using a lagged fibonnacci generator, can  this seed be found by brute force?
Assuming by "digit" you mean "base-10 number"...

There are $10^4$ different combinations. 

$\log_2(10^4) \approx 13$

It would require no more than $2^{13}$ guesses to guess every single possible seed. This assumes that the seed is uniformly random.

Even using python on relatively modest hardware, it takes .001 seconds to iterate over all the possible seeds.

So yes, it is easily guessable. 

You would require at least $2^{64}$ cost to become merely inconvenient rather than trivially guessed. 

The minimum size for it to be considered secure is 128 bits. That would correspond to roughly 39 uniformly random base-10 digits.