Martin is asked to find the probability of getting 1 head and 3 tails on 4 coin tosses. this is a simple event

The possible outcomes for 1 HEAD and 3 TAILS are

HTTT
THTT
TTHT
TTTH

Assuming the coin is a fair coin, we have:
Probability of tossing head is 0.5
Probability of tossing tail is 0.5

Probability of tossing the coin four times for each outcome combination listed above is

HTTT = 0.5×0.5×0.5×0.5 = 0.0625
THTT = 0.5×0.5×0.5×0.5 = 0.0625
TTHT = 0.5×0.5×0.5×0.5 = 0.0625
TTTH = 0.5×0.5×0.5×0.5 = 0.0625

Hence, probability of getting three tails and one head from 4 tossing is 
4×0.0625 = 0.25