Hi Simon,
Welcome to the world of AG 4 - I think you'll grow to love the runners so I'd reccommend dropping at least 1 thrower and using some spare cash to upgrade to get another gutter runner on your starting team - They get better the more you have of them, and it's a lot easier for an opponent to counter 2 than it is with 4.
The Skaven's strength is it's speed so I'd save for the 4th as soon as I can - I would suggest using the Very Long Legs mutant as a proxy gutter runner if you don't want to get any more models.
The difference between picking up the ball and making a short pass with the thrower with his inbuilt rerolls v.s. the AG 4 of the gutter runners is quite small... The gutters are much more mobile though, with higher movement and dodge, so ideally you can have at least 1 back as a ball handler and 2 waiting for passes/handoffs a few squares into the opponent's pitch.
They're much more survivable in starting games due to the dodge skill, where less opponenents have got block or tackle, so are easier to skill up.
I'd also start with either an apoth
or a 12th player, and squeeze a 3rd reroll in if I could... I know it's risky if you have a bad first game but it pays off longer term. Remember a journey rat does just as good a job of dying as the team's line fodder!
I'm looking forward to seeing them on the pitch!

Thanks
Steven