My pup is now 4.5 months old and still erratic at coming back. A a result I don't dare to let her off the lead when we're out, although I can tell she'd love to run a bit more freely (she's on an extendable lead). In the garden she comes back to me about 80% of the time, but mostly ignores my husband, and there are times when she just totally ignores me. If I go to get her she runs away and the situation gets worse so I avoid doing that where possible. What to do? Or is it still too young to expect more?

none of my puppies are off the lead freely untill a year old.
for two reason
1 to get all the training in first walking to heel,sit .wait ect on the lead.plus you can do your recall on a long lead.but be care full some one told me not shore if it ture dog lead in should not be over a 18 inches long by law.( i was told this]BEACUSE OF THE DANGER SIDE.
2 not to over excires your before a yesr due to free running.
she still every young had you tip bit treat to award her when she comes back to you.
do you call by her name from say come and praiser when she close to you.

i'd get the recall perfect at home before you try and do it while out. i lways use the word 'come', you can practise this in the house as well as the garden.
if your in a different room just call out 'come' and repeat the word as you see her coming to you, so she begins to associate the word with the action and when she gets to you make her sit and reward with a treat. it's good to get her to sit as when you do let her off the lead when out it gives you chance to put the lead on.
the other way to practise is to put her on the lead in the garden and slowly pull her to you saying the command word and then treat her.
once she learns the word and that coming gets a treat or toy then she'll soon get the hang of it but at her age it takes time and patience.
if you can don't go and get her always make her come to you, if you chase her it will become a a game,better to run the other way and say the command in a high pitch voice and in an excited way. usually thats enough for them to want to see what your up to.
once she's got the hang of it and you can let her off on walks make sure your in a safe place so that if she does run off you don't need to panic and keep calling her using the command word every couple of minutes then give her a treat and let her run off again. if you only call her when you want to put the lead on she'll learn not to come back as this means the end of fun.
if you know someone else with a dog who has good recall this can help if you walk together as your pup will be interested in playing and following the other dog and it will give you more confidence that she will come back.
just take things slowly and don't expect too much to soon and your be fine.      

I guess I'm lucky to have 3 dog savvy kids which made recall training fun. As a pup we'd all take a mixture of her favourite treats and spread out around the house or garden and take it in turns calling her name and the command 'here'.  She'd hunt out whoever was calling, be rewarded and then the next person would be calling and of she'd go again. My partner wasn't always there to join in the game and her recall for him isn't quite as good, but if my youngest son calls her then she's off like a shot - think he had trouble limiting it to one treat ;)

Tracking lead from ebay 100 foot long about ten pounds for practise, big open field and a whistle for recall and bum bag of treats worked a treat for my dog who was a nightmare

You're right to avoid going to get her - 'chase' is a great game! If you turn and run the other way - away from her - she's far more ikely to follow you. I always let mine off the lead as soon as possible after they've finished their injections, when they're still young enough to be nervous of being left behind. Once they gain confidence it becomes more difficult. Do you call her a lot? More than once teaches her that she can ignore you in safety. It might be worth whistle-training her instead of calling.