Is a temperature of 37.4 okay post breast augmentation?

Many thanks for your question an congratulations on your very recent breast augmentation.
In answer to your question a temperature of 37.4 is not completely out of the ordinary but needs to be in context.
It is not uncommon for temperature recordings to go up after a general anaesthetic, although at 4 days post-surgery I would expect it not to be raised.
Various factors such as where you are in your monthly cycle, if your breasts are red/hot/swollen, if you are experiencing shivering and/or feeling unwell will all contribute to assessment and clinical decision making. 
I would therefore recommend that if you have any doubts or concerns then you go and see you operating surgeon for review and reassurance.
I wish you well.
Best wishes,
Marcus