Scourges and heavy weapons: 4 in 5

3 haywire + 1 hl,
3 hay + 1 blaster
2 hay + 2 hl
3 hl +1 hay
or
3 hl + 1 blaster
Which would you recommend?

I wouldn't recommend mixing their weapons.

Think of the roll you want a unit to accomplish and gear them for that roll.  
You want to kill tanks then haywire is a great choice in that it has a solid range and will bust vehicles quite easily. But they won't help you kill MC's or infantry very well.  the range also helps keep them out of harms way.

Heat lances are amazing for popping armour as well, but you have to get really close to do any real damage. maybe 10 scourages with a WWP character.  great at vehicle busting, temrie/infantry and MC killing.

The Blaster is in my opinion is the in-between of the haywire and heat lance. They are nice in that they can hurt MS's/powers armour/termies very well and are pretty good for taking out vehicles. You don't have to get a close to a vehicle as a heat lance which is nice, but you have to get closer then the haywire.

Four haywires. Convert the fourth one if you need to, they have to be the best choice for this unit. The stats being unchanged means they can flap flap flap 12" then fire 24" - that's a big range for such a potent weapon. With a blaster, you're having to get closer, with a heat lance closer still. I love heat lances, but I wouldn't give them to anything other than Reavers unless I didn't have the bits. If four haywire blasters really aren't an option, I would go with three haywires, one heat lance. Some bright spark on here did the maths a while ago, and a heat lance has a better chance of penetrating than a blaster (obviously not factoring in such huge variables as whether it can get into melta range). I take heat lances when it's a choice between those and blasters. I take blasters when it's a choice between those and shredders. I take shredders when AH HAHAHAHAHA I couldn't finish my sentence.

Thanks for the conversion ideas guys. I may look into that!
I'm getting the book tonight. Scourges are going to be my next big project. I'll aim for Haywire Blasters, probably a unit with Blasters (as you guys don't seem to be too keen on the melta weapons), and maybe a larger unit with Shardcarbines.
It's going to be deepstriking ridiculousness all over the place!

I'm pretty new to seventh edition and jumppack units. Is there any way to deepstrike them, kill the armour and get them to safety somehow? I assume you can't deepstrike them into cover, so suicide unit all the way?

if you take haywire blasters, they won't blow the vehicle up, so it remains on the table as LoS blocking terrain. Plus the 24" range of haywire blasters means you don't have to get too close...

Ah right, thank you for explaining. I'm looking forward to this!

If it's open-topped, you have an extremely small chance of exploding it - just bear that in mind. For those asking about conversions, I did mine with a heat lance and the end of a shredder. Obviously different up close, but looks fine from a distance and is pretty obvious.

Question for the thread: I currently have 3 carbines and 2 haywire blasters. Were I to purchase another box to make a unit with 4 haywire blasters, what should I do with the other 3 (they're going to have at least 2 carbines in the unit)?

I'd put heat lances or blasters on them, although 4 splinter cannons wouldn't be a terrible idea (and might be slightly cheaper than 2 venoms)

Ehm, sorry to disappoint you Burnin' Eye, but the SC is 15pts for a salvo 4/6 weapon. You don't want to pay 93% extra per model for 1 extra splinter shot (It's not like you're gonna have them camping, right?).