Alright, heres the story:

I'm at the edge of buying a Crate 4x10 cab

But despite my broad horizons of knowledge, I'm not sure what head I should buy for it...

The guy [who i'm buying it from] does not know how many watts they are. all he knows is that they say " BV-4125 " on the back.

[but if they turn out to be ****ty i'm gonna replace them]

So I'm guessing they are 25 watts each.

So do i need to buy a 100 watt head for a 100 watt cab ? is that how it works ?

and would it be better to get a crate head ? or does it not matter ?
Oh, yes sorry, I will be using it for guitar [G-400]
I'll be using it for in my room and for jamming with people and possibly small gigs
Tbh, I'd recommend that you try a 4x10 and 4x12 in your local guitar shop. You might find that the 10" speakers don't deliver enough bottom end for your needs.

It doesn't really matter who the head is made by, as long as its power rating isn't higher than that of the cab, and the cab runs at the right impedance.

Are you going to get a solidstate or tube head?
Probably says on the speakers

BTW, look at the model number. I'm betting it's a 4x12". It's a Blue Voodoo cab, so I'd imagine it has high power handling.
Now it's 1984
Knock knock at your front door
It's the suede denim secret police
They have come for your un-cool niece
Just so's you know, you're doing this back to front, you choose a cab to match the amp, not the other way round.

If the amp is a combo, then you don't need to look any further, if it only comes as a head though you'll need to source a suitable cab.

So, the first thing you need to do is set yourself a budget, then you need to decide what kind of tone you want from your amp based on what styles you play, then you start looking for an amp.
Actually called Mark!

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i wanna see a clip of a recto buying some groceries.

hmmm...I'm not sure if im getting tube/solid state, depends on the deal i can get.

yeah i know im doing it the other way around
but this guy is selling it for $100
which i can probably bring down a bit
so yeah.

Well my budget is as low as possible -$380
i play metal, rock and i dont mind a good accoustic sound