Here's some more information on using H4s in a Datsun 1200.

Headlight Wiring
* Relay
** Harness Kit
** DIY

It has pictures and such

I thought that would be obvious dd, sorry should have mentioned that

H4s are considerably brighter because they use considerably more current!

It will overload the existing wires. Relays won't fix the problem, you need larger wires with the relays!

if this helps, wouldnt be more then about an hours worth of work

http://www.humanspeakers.com/audi/headlight-simple.htm

So if i put relays in can I use the higher watt globes?

If I just change the globes to 50/55 will it be ok without relays?

Don where's the love???? Your not my wife- show me some loving....

I learnt this lesson before it happened. Luckily. I used 60:90 globes and was very lucky.

What wattage are the globes? Your standard circuit is not designed to take more than 50/55 globes. If you have fitted higher wattage globes you can kiss your fusebox goodbye. I hate to say it, but as Mik said, Relays are the answer.

Just like he said ^^^^, unless there is a short circuit in the wiring but that should blow the fuse straight away.

Need relays- also its more likely the wire sheaths melting or the actual fuse box

Get onto it ASAP.

Might also benefit taking the fuse box out and cleaning the connectôrs
From the rear.
Make sure you label where the wires go into the box too.

I bought aftermarket lights off ebay that take h4 globes and when they are on I can actually smell the fuse getting hot while im driving until it eventually melts.
Is there an easy fix?
All my wires are stock, no relays or anything.
Ive done a search and everything keeps coming up with mixed results.