'f' and "s' are usually developed by about school age (ie. 5 years), 'l' can take a little longer. However, the examples of words you gave (clock, snake) start with 2 consonant sounds (blends) and are therefore harder to say and can take up to 7-8 years of age to fully develop. So, at 4 years of age sounds very normal to me. You, and strangers, should be easily able to understand what a 4 year old is trying to say but words can still be said with sound errors. The sounds you have mentioned are typically still developing for some children at 4 and this is perfectly within the range of normal.