What worked for us was to go for slow short bike rides in places where the 4yo could ride their own bike as 'one of the group' - obvs nowhere steep, busy or bumpy.

Amazingly quickly, their stamina and confidence increase to the point where you can't keep up with them (ours was 7 when that happened)

We tried tagalong, trailgator etc but they get frustrated if
they are missing the independence and control, which imo are the best part of cycling when you are 4. Like going hiking, you do have to go at their pace, but you can sneak out for a grown-up ride later. Frequent snack stops help too.

We had better experience with a balance bike than with stabilisers but that's another discussion.

Posted By: WillInAberdeenAmazingly quickly, their stamina and confidence increase to the point where you can't keep up with them (ours was 7 when that happened)

This is very true and is probably the thing to work towards long term - but long term will be months/years

If you need to go distance or on a busy road in the meantime a tag along or bike seat work well.

Bike seats have a weight limit (20Kg) and can make the bike hard to handle when they're in it. If she's already you may want to skip a seat.

If she can ride her own bike she should hold onto a tag along in the same way but as with everything new I'd practice somewhere quiet a few times first.

cheap tag alongs can have poor quality link-ages that jerk you them around a bit - if you're going tagalong I'd buy a decent one.

OK, a bit more expensive than a tag along but so much more fun as you can talk to the offspring and point things out along the way:
http://schoolruncentre.co.uk/tandem-onderwater-adult-and-child-tandem/
On top of that, judging from how she got along on her own bike I tend to believe our daughter got a sense of how to go about in traffic from having had the best seat in the house for years.
It has top quality components and tandem wheels & hubs so no trouble carrying panniers full of shopping in addition to the passengers. It also means the trade in value was substantial. And no, I don't have any personal interest in promoting these things except for getting people, both big and little, out of their cars.

Posted By: VictorianecoAny recommendations for a tag along and what about harnesses / belts?

We brought a cheap one and the linkage caused the tag along to jerk from side to side - wasn't great but did teach my daughter to hold on tight!

Getting your daughter a lightweight quality bike makes a big difference. Brands like easy rider, frog and Isla make light bikes with good components for little people rather than adult components which the cheap ones often
have. A trailgater is good, a metal bar that fixes your bike to theirs, as they can ride on their own but when they're starting to struggle you can just hook them up to your bike to speed things up!

Yes, she loves her bike at the moment but she's certainly not capabale of riding by herself on pavements etc and we live in quite a hilly area. She's only 4 rememeber

So yes a towing type mechanism may be best and just unlink her when we get to the flats.

Any recommended brands for a tow bar?

I've picked up a Trailgator and set it up last night (one of my skewers was seized so need to pick up a new one)

There is a rod that connects the bar to the bike being trailed that seems to stop the handlebars turning, this seems to be flimsy and no doubt easily removed by my daughter if she decided to flip it off. This part:
http://www.trailgatorspares.co.uk/parts/trail-gator-stabiliser-rod-parts-aa-bb-cc/5/1/all

My guess is that this isn't essential? As the front wheel is lifted, turning that wheel makes no difference to the safety aspect of running it perhaps adding some extra stability? Need to clarify it before I set off later