Account type. For the first $5500/yr, you’ll probably want to pick between Roth IRA and Traditional IRA. Since you have very little saved right now, a Roth could also function as an emergency fund. If you’re a W-2 employee (as opposed to a 1099 contractor), you don’t have any other options for a retirement account, so any additional money (above the $5500/yr) could go into a regular taxable brokerage account. If you’re 1099, you could consider SEP IRA (maybe even instead of or in addition to Roth, although I don’t recall the rules).
Investment allocation. Start (and quite possibly finish) with a Lazy Portfolio. Use ETFs (instead of mutual funds) where possible, since they don’t have a minimum and have the same low expense ratios as the cheapest mutual funds.
Brokerage. If you follow a Lazy Portfolio approach, it’s probably easier at Vanguard.
And you really should have a bigger emergency fund than $4K. If you lose your job, you could lose your house pretty quickly after that.