Interference??

[emaxos]

IMHO you made the right call. Runners should not run into a tag. By staying around 1st base, R1 may have put herself in jeopary but is not required to advance unless the BR attains 1st base. F4 had options to get one or two outs but was probably just dumbfounded when R1 didn't run. R1 was not required to provide a clear throwing lane for F4 and unless she took action to get in the way, there's no interference.
If both runners ended up on first, R1 is out because the BR has a right to 1st base.
That's my "unofficial" opinion, anyway.
Let's see what the experts say when they get up.